Glossary of SEO Terms:

Anchor Text: "Linked Text" Anchor text is the text that you click on to activate and follow a hyperlink to another web page or another web site.
Alt Tag : Alt tag refers to the text that is associated with an image. This originated from when web browsers were not universally available as "Graphical Browsers" so the "Alt text" was a description used to describe the image. Now adays it is also used in the "mouseover" text popup that appears when you move your mouse over a graphical image.
Cloaking: Method by which specific content is served up to the search engine spider that is different then what the normal surfer sees.
CPC: Cost per click. This is a frequent term used in PPC terminology. It refers to the cost associated with each click.
CPA: Cost per Acquisition or also known as Cost Per Sale. This is typically an average dollar amount to the total cost in clicks it takes to convert to a sale.
Conversion Ratio: The conversion ratio is the average number of visitors who visit the site who make a purchase or perform some sort of action. (register as a customer, etc). Conversion ratios vary by the type of traffic you are getting. More "Targetted" traffic typically means higher conversion rates.
Doorway pages: Pages setup specificly for search engines. Once the visitor reaches the page, they are then redirected to another website.
Hits: This generally means ALL requests from a webserver including requests by a web browser for html pages, jpeg's, gif's and other images. Hits is a phrase often thrown around but is generally not very meaningful in quantitfying search engine traffic.
PPC: Pay Per Click.
PageRank: Formula developed by Google to determine a web pages "inbound link ranking" Often referred to as "PR" value.
PageViews: Number of times your webpage was viewed. Includes duplicate views by the same visitor.
ROI: Return on Investment. A quantitative analysis of investment in advertising and marketing budgets and the resulting return on the investment.
SEO: Search Engine Optimization.
SEM: Search Engine Marketing.
SERPS: Search Engine Result Pages.
SPAM: Unscrupulous or unethical means of inflating results. Usually deteriorates the quality of listings and often results in penalties or being banned from a search engine.
Unique Visitors: Total number of unique visitors to your website or web page.
Cloaking: Displaying different content to search engines and searchers. Depending on the intent of the display discrepancy and the strength of the brand of the person / company cloaking it may be considered reasonable or it may get a site banned from a search engine. Cloaking has many legitimate uses which are within search guidelines. For example, changing user experience based on location is common on many popular websites