Keyword
A word typed into a search engine in order to find web pages that contain that word. A web page can (and should be) optimized for specific keywords/phrases that are relevant to the content on that page.

Keyword Density
A measure of the percentage of words on a page that are specifically chosen keywords. When a user enters a query, search engines display a list of pages containing the search terms. These are ranked based on (amongst many things) the percentage of words on a page that are similar to the words used in the query (keyword density). When keyword density is inflated artificially, it is often referred to as keyword stuffing.

Keyword phrase / Key phrase
Text on a web page that is exactly or almost the same color as the background. The use of invisible text to load a page with keywords was once a popular SEO technique, but search engines can now detect invisible text and penalize sites that use it. Although there are examples of sites that use invisible text and "get away with it" on Google, the general consensus is that it is not worth the risk. The same results can usually be achieved by working the keywords into the visible body text.

Keyword Stuffing
Excessive repetition of keywords in an attempt to artificially inflate keyword density and improve a page's ranking. Keyword stuffing is easily detected by search engines and pages that use this technique are penalized.

Keywords Meta Tag
An HTML meta tag that lists all of the main keywords and key phrases that are contained on that web page. Some search engines use the keyword meta tag to help rank web pages in their databases. Google does not.

Keyword Targeting
The practice of optimizing certain pages of a web site to rank well in a search for specific keywords. Keyword targeting is generally considered vital to effective SEO