Glossary of search engines

And especially the language used by webmasters in dealing with search engines.

Link from another site on yours. See the article: All you need to know about backlinks.
Black hat
Fraudulent methods to improve the position of a site, such a farm links, satellite pages.
A blacklisted site is no longer indexed by search engines.
Bounce factor
This is the percentage of visitors who leave the site without to read a second page.
It should be as low as possible because it measures the interest in the site content. But as a quality criterion it is questionable because visitors who come through search engines and are looking for the answer to a question have no reason, it they got the answer, to view other pages.
That is showing to search engines a page different from that visitors see. It is prohibited by the guidlines and the site may be banned from the search engine index. Note that serving HTML to robots while users see Flash or images is viewed as a cloaking trick: the HTML must be viewed by users too.
The JavaScript content and the noscript one must be the same.
A doorway page is a page made specifically for search engines, which usually contains significant quantities of links to a site, to emulate a popularity it has not, or internal links to make anchors.That leads to the removal of the site from the index.
An HTML sitemap (with links to the same site) is not considered doorway if its design makes it convenient for visitors.
Duplicate content
There is duplicate content when two pages are identical on one site, where a site takes the pages from another site, but also for robots of search engines, when two different URLs can drive them on the same page. This is particularly the case when we redirect two domain names on the same site without using 301 redirection.
The duplicate content penalizes a site in many ways as detailed a post on the blog Google.
EMD (Exact Matching Domain)
A name that contains exactly the keywords of the topic of the  site. For example, exotic-birds for a site dedicated to these species.
GWT (Google Webmaster Tools)
Panel provided by Google to get statistics and interact with the engine.
A theoretical extension to the PageRank, and that could prevent manipulations by an algorithm which classifies a page solely on the basis of links from authoritative sites. This is partially used by search engines according to the Google's patent.
HTTP status codes
When a request is made to read a page on a server, an HTTP status code is returned. The main codes are:
- 200: OK, the URL is reached and the page is read.
- 301: redirected permanently. This may be achieved with a PHP function or the .htaccess file.
- 302: redirected temporarily (avoid to use such redirection).
- 404: not found.
Keywords are the foundation of SEO, as they are the basis of search engine and source of traffic. They may be choosen for traffic or for money. In the second case they must be relevant with business.
The keyword density is an old measure for SEO.
An expected progress for search engines to identify the root of words and retrieve pages sharing same roots of words.
Meta tag
A meta tag provides informations about a page.
Over optimization
Optimizing a website for search engines is to facilitate indexing of the content, including by integrating relevant keywords, but spam is to put more than necessary. In fact, over-optimization can relate to all aspects of SEO.
PageRank or ranking
Rank in the Google hierarchy, a score given to a page dependant to backlinks and external links. The more it is raised, the more the page tends to be at the top of the list of answers from search engines (but many other criteria, are taken into account).
The name comes from the Pogo stick, a stick with a platform and a spring to move by jumping. In SEO, the name is applied to the action for a user to click on a link in results page, come back and click on another link.
ROI (Return On Investsment)
Measurement of how much you earn from each invested dollar.
A site is known sandboxed by Google when its rank page seen to be allowed a rank of 0. It remains indexed, but at the end of the list of results and thus not seen by anyone. This effect can apply to all research that the site answers, or to only a group of keywords in particular.
A new website enters also the sandbox, this is applied by google against spammers that create new sites only to manipulate search engines.
Search Engine
Online tool to find information or a site on the Web (or local engine).
SERP (Search Engine Results Page)
Page of results in answer to a search query.
SEO (Search Engine Optimisation)
Techniques to make Web page in such a manner they are best indexed by search engine and get a better position in results.
They consist in architecture of pages, keywords, links and renow.
Map of a site, concretely, a list of URL of the pages which should be indexed, and other information for robots.
Name given by Google to the text under the title of a page in results of requests. It may come from the description meta, from Dmoz of from the contents of the page.
Processes used to cheat search engines, therefore to privilege its site without regard to the contents, but by making profit of the way robots of search engines work instead. For example, accumulation of keyword, text hidden (white text on white background), pages of links towards its site, etc. This lead to the blacklist.
Uniform Resource Locator, addresses of a site including the domain name and the name of the file, example:
UX (User eXperience)
What the user feels when visiting a page, its accessibility and ease of use if any, has become a ranking factor more important than the content because it is easier to evaluate than the content (through the user's behavior, that search engines are able to detect).
White hat
This is the opposite of black hat, so legitimate optimization of a website, in the rules as issued by the guidelines of the engines.

See also the glossary of web site creating.