Click-through Rate (CTR): The percentage of those clicking on a link out of the total number who view the link or text ad.

Cloaking: In terms of search engine marketing, this is the act of getting a search engine to record content for a URL that is different than what a searcher will ultimately see. It can be done in many technical ways. Several search engines have explicit rules against unapproved cloaking. Those violating these guidelines might find their pages penalized or banned from a search engine's index. As for approved cloaking, this generally only happens with search engines offering paid inclusion program.

Contextual Link Ads/Inventory: To supplement their business models, certain text-link advertising networks (like Google) have expanded their network distribution to include “contextual inventory”. Most vendors of "search engine traffic" have expanded the definition of Search Engine Marketing to include this contextual inventory. Contextual or content inventory is generated when listings are displayed on pages of Web sites (usually not search engines), where the written content on the page indicates to the ad-server that the page is a good match to specific keywords and phrases. Often this matching method is validated by measuring the number of times a viewer clicks on the displayed ad. These ads typically do not perform as well as traditional text ads on search engines, but the lower cost justifies the expense.

Conversion Rate: The relationship between visitors to a web site and actions considered to be a "conversion," such as a sale or request to receive more information. This metric is often expressed as a percentage.

Cost-per-Click (CPC): System where an advertiser pays an agreed amount for each click someone makes on a link leading to their web site. Also known as PPC or paid listings.

Cost-per-Thousand (CPM): System where an advertiser pays an agreed amount for the number of times their ad is seen by a consumer, regardless of the consumer's subsequent action. This term is heavily used in print, broadcasting and direct marketing, as well as with online banner ad sales. CPM stands for "cost per thousand," since ad views are often sold in blocks of 1,000. The M in CPM is Latin for thousand.

Crawler/Spider/Robot: Component of search engine that indexes web sites automatically. A search engine's crawler (also called a spider or robot), copies web page source code into its index database and follows links to other web pages.

Directories: A type of search engine where listings are gathered or reviewed by humans, rather than by search engine crawlers. In directories, web sites are often reviewed, summarized in about 25 words and placed in a particular category. The largest and most popular directory site is Yahoo!

Doorway/Landing/Gateway/Bridge/Jump Pages: A web page created expressly in hopes of ranking well for a term in a search engine's organic/non-paid listings and which itself does not deliver much information to those viewing it. Instead, visitors will often see only some enticement on the doorway page leading them to other pages, or they may be seamlessly redirected to a real page within the existing web site. With cloaking, visitors may never see the doorway page at all. Several search engines have guidelines against doorway pages, though they are more commonly allowed in through paid inclusion programs.

eBuzz: Also referred to as guerilla marketing or grassroots marketing. See “Online Public Relations.”

GoogleDex: GoogleDex is the score given to a term based on the number of pages that Google has indexed and posted as results for that term.

Grassroots Marketing: See "viral marketing."

Guerilla Marketing: See "viral marketing."

Index: The collection of information (contained in a large database) a search engine has that searchers can query against. With crawler-based search engines, the index is typically copies of all the web pages they have found from crawling the web. With human-powered directories, the index contains the summaries of all web sites that have been categorized.

Inbound/Back Link: A text or graphical hyperlink from one site to another. Google and other search engines’ algorithms consider a site’s popularity based on the quality and quantity of inbound links from relevant third party sites to help determine search positioning. See “Link Popularity.”

Keyword Density: The frequency of repetition of a given keyword or phrase within body text on a web site. The higher the frequency (measured in percentages) the greater the likelihood of a higher ranking in search results. In Anvil’s case, you’ll notice a higher density of the phrase “search engine marketing” within the web site, as that is the most relevant phrase used to describe the service offering.

Link Development: The act of requesting or securing inbound links to your web site. Also see Link Popularity and Inbound/Back Links.

Link Popularity: A raw count of how "popular" a page is based on the number of backlinks/inbound links it has. It does not factor in link context or link quality, which are also important elements in how search engines make use of links to impact rankings.

Listings: The information that appears on a search engine's results page in response to a search. See “Results Page.”

Meta Search Engine: A search engine that gets listings from two or more other search engines, rather than through its own efforts.

Meta Tags: Information placed in a web page not intended for users to see but instead which typically passes information to search engine crawlers, browser software and some other applications.

Meta Description Tag: Allows page authors to say how they would like their pages described when listed by search engines. Not all search engines use the tag.

Meta Keywords Tag: Allows page authors to add text to a page to help with the search engine ranking process. Not all search engines use the tag.

Meta Robots Tag: Allows page authors to keep their web pages from being indexed by search engines, especially helpful for those who cannot create robots.txt files. The Robots Exclusion page provides official details.

Online Public Relations: The use of traditional public relations strategies and tactics to garner coverage for a company or Web site via online media: ezines, online publications, newsgroups, blogs, threaded forums and chat rooms.

Organic/Natural Listings: Listings that search engines do not sell (unlike paid listings). Instead, sites appear solely because a search engine has deemed it editorially important for them to be included, regardless of payment. Paid inclusion content is also often considered "organic" even though it is paid for. This is because that content usually appears intermixed with unpaid organic results.

Outbound Links: Links on a particular web page leading to other web pages on a different domain.

Paid Inclusion/Pay-for-Inclusion (PFI): The act of purchasing the ability to be indexed by search engines. Unlike PPC, position within search results are not guaranteed, but unlike organic SEO, PFI guarantees a level of frequency in indexing and enables optimization and submission of large numbers of pages within a site. The end result is ideally a higher position in search results for larger, database-driven sites.

Pay-per-Call: The ability to track offline sales through unique toll-free phone numbers. Currently available on FindWhat and CitySearch properties, this service is ideal for offline-based businesses like plumbers, contractors and other service industries.

Pay-per-Click (PPC): Stands for pay-per-click. See “Cost Per Click” and “Paid Placement.”

Paid Listings: Listings that search engines sell to advertisers, usually through paid placement or paid inclusion programs. In contrast, organic listings are not sold.

Pay-for-Performance: Term popularized by some search engines as a synonym for pay-per-click, stressing to advertisers that they are only paying for ads that "perform" in terms of delivering traffic, as opposed to CPM-based ads, where fees are based on impressions or views instead of clicks.

Paid Placement: Advertising program where listings are guaranteed to appear in response to particular search terms, with higher ranking typically obtained by paying more than other advertisers. Paid placement listings can be purchased from a portal or a search network. Search networks are often set up in an auction environment where keywords and phrases are associated with a cost-per-click (CPC) fee. Overture and Google are the largest networks, but second tier players like FindWhat are gaining in popularity as CPC prices increase. Portal or site sponsorships are also a type of paid placement.

Rank: How well a particular web page or web site is listed in a search engine results. Generally, sites on the first page (or within the first 10 listings) generate significant visibility and traffic. Overall, saying a page is "listed" only means that it can be found within a search engine in response to a query, not that it necessarily ranks well for that query. Also called position.

Reciprocal Link: A mutually-agreed upon link exchange between two sites. See “Link Development.”

Results Page: The page that is displayed after a search phrase is typed into a search engine. Also referred to as search engine results page or SERP.

Robots.txt: A file used to keep web pages from being indexed by search engines. The Robots Exclusion page provides official details.

Return-on-Investment (ROI): Historically associated with sales and marketing efforts; when applied to SEM efforts, refers to numerical, percentage or ratio of revenue generated over total cost of activities. ROI typically factors in paid placement and associated management costs, but a more detailed analysis may factor in profit (true cost). If ROI is measuring paid placement only, it is typically referred to as return on ad spend (ROAS).

RSS Feeds: Real simple syndication (RSS) is a relatively new and easy way to distribute content via the Internet. For email marketers, it is a way to distribute messages while avoiding spam filters. Typical applications include email newsletters, blogs or even Web sites. Similar to newsgroups, RSS feeds require a special “reader” like Bloglines or NewsGator to view messages.

Search Engine: Any service generally designed to allow users to search the web or a specialized database of information. Web search engines generally have paid listings and organic listings. Organic listings typically come from crawling the web, though often human-powered directory listings are also optionally offered. Top tier search engines include Google, MSN, Teoma and Yahoo!

Search Engine Marketing (SEM): The act of marketing a web site via search engines, whether this be improving rank in organic listings (search engine optimization), purchasing paid listings (PPC management) or a combination of these and other search engine-related activities (i.e. affiliate programs, shopping feeds or link development).

Search Engine Optimization (SEO): The act of altering a web site so that it does well in the organic, crawler-based listings of search engines. In the past, has also been used as a term for any type of search engine marketing activity, though now the term search engine marketing is more commonly used as an umbrella term.

Search Engine Positioning (SEP): Synonymous with SEO, search engine positioning is the act of altering a web site to perform well in organic or natural search results.

Search Engine Submission: The act of submitting specific URLs to popular search engines like Google, MSN and Yahoo! to ensure the web page gets spidered and indexed.

Search Terms: The words (or phrase) a searcher enters into a search engine's search box. Also used to refer to the terms a search engine marketer hopes a particular page will be found for. Also called keywords, query terms or query.

Shopping Search/Feeds: Shopping search engines allow shoppers to look for products and prices in a search environment for rapid and easy comparison. Premium placement can be purchased on some shopping search indices via “XML feeds.”

Site Optimization: The act of fine-tuning web site content and code to perform well in search engine results. See “Search Engine Optimization.”

Spam: Any search engine marketing method that a search engine deems to be detrimental to its efforts to deliver relevant, quality search results. Some search engines have written guidelines about what they consider to be spamming, but ultimately any activity a particular search engine deems harmful may be considered spam, whether or not there are published guidelines against it. Examples of spam include the creation of nonsensical doorway pages designed to please search engine algorithms rather than human visitors, or a heavy repetition of search terms on a page to increase keyword density. Also referred to as spamdexing.

Submission: The act to submitting a URL for inclusion into a search engine's index. Unless done through paid inclusion, submission generally does not guarantee listing. In addition, submission does not help with rank improvement on crawler-based search engines unless search engine optimization efforts have been implemented. Submission can be done manually (i.e., you fill out an online form and submit) or automated, where a software program or online service may process the forms behind the scenes.

Viral Marketing: Any marketing technique that induces Web sites or users to pass on a marketing message to other sites or users, creating a potentially exponential growth in the message's visibility and effect. See article, "Viral Marketing: Miracle Cure or Common Cold?" by Kent Lewis for more information.

XML Feeds: A form of paid inclusion where a search engine is "fed" information about pages via XML, rather than gathering that information through crawling actual pages. Marketers can pay to have their pages included in a spider based search index either annually per URL or on a CPC basis based on an XML document representing each page on the client site. New media types are being introduced into paid inclusion, including graphics, video, audio, and rich media. These feeds are commonly used for Shopping Feeds.

Definitions above draw on multiple sources, as well as original writing. Content used is copyrighted, and remains property of its respective owners: I-Search Digest, Webmaster World Forums, SearchEngineDictionary.com, SearchEngineWatch.com, and WhatIs.