Algorithm – A decision making logic utilized by search engines to decide what results to serve and how to rank them.
ALT – Text added in the code to describe images. ALT has value for SEO as search engines use its content instead of attempting to understand the purpose of the image.
Anchor text – Text contained within the link. This is one of the significant search engine signals and has impact on rankings.
Backlink – Refers to external link pointing to the observed website.
Canonicalization – Process of establishing an original version of a document which may have multiple URL variations.
Duplicate Content – Content present on more than one URL.
External Links – Links to pages not hosted on the domain of the linking site.
Follow – Part of the rel attribute which instructs robots to follow links on the document.
Googlebomb – Method of influencing rankings by massive amounts of inbound links with targeted anchor text with typically undesirable results.
H tags – Headings in documents (H1, H2, H3…etc) used to structure the document in meaningful topics and sub topics.
Inbound Links – Links pointing to an observed website from external websites.
Joomla – Content management platform similar to WordPress.
Keyword – Single word observed in terms of rankings and content.
Keyphrase – A phrase observed in terms of rankings and content which may contain multiple keywords.
King, the content – SEO industry professionals will often refer to content as being the king due to the fact that search engines value good content and wish to rank it better. Content will also be the factor that decides how well your pages will be linked by others.
Link – Short for Hyperlink. Means of connecting one online document to another. Links are looked at by search engines and represent one of many search engine ranking factors.
Meta tags – Data used in the source code of websites to give search engines additional information.
Nofollow – Part of the rel attribute which instructs robots not to follow links on the document.
Outbound Links (Outgoing links) – Links pointing away from an observed website towards external websites.
PageRank – Metric introduced by Google to measure the importance of the page on the web.
Panda – Google algorithm update which included changes on content evaluation including value, ads and user perception. Panda was directed at an ever increasing number of low quality pages created for ads or link manipulation. (Thanks: @hyderali_)
QDF – Abbreviation: Query Deserves Freshness. Search engines treat fresh content differently than old documents and will adjust the rankings to make certain searches more timely.
Rankings – Positions of a website in search engine result pages for one or more search terms.
Relevant backlinks – Meaningful inbound links from websites of related topic.
SEO – Abbreviation: Search engine Optimisation (or Search Engine Optimization). SEO is a marketing discipline which studies the way search engines analyse and rank pages and applies the best practices to web content in order to achieve better rankings.
TrustRank – A metric similar to PageRank introduced to describe the level of document or domain trust from a search engine.
Unique Content – Original content not present on any other URL on the internet.
Unique Visits – Website traffic which excludes multiple visits by the same user.
Visits – Website traffic which includes all visitors including multiple visits by the same user.
WordPress – Popular content management platform.
X – Software by SENuke which automates SEO process. Typically not endorsed or recommended by search engines or SEO professionals.
YouMOZ – Open blog platform by SEOMoz which accepts entries from SEO professionals not employed by SEOMoz.
Zeal – Popular web directory.