Keyword density refers to the ratio (percentage) of keywords contained within the total number of indexable words within a web page.

The preferred keyword density ratio varies from search engine to search engine. In general, I recommend using a keyword density ratio in the range of 2-8%.

You may like to use this real-time keyword analysis tool to help you optimize a web page's keyword density ratio.

Keyword Frequency

Keyword frequency refers to the number of times a keyword or keyword phrase appears within a web page.

The theory is that the more times a keyword or keyword phrase appears within a web page, the more relevance a search engine is likely to give the page for a search with those keywords.

In general, I recommend that you ensure that the most important keyword or keyword phrase is the most frequently use keywords in a web page.

But be careful not to abuse the system by repeating the same keyword or keyword phrases over and over again.

Keyword Prominence

Keyword prominence refers to how prominent keywords are within a web page.

The general recommendation is to place important keywords at, or near, the start of a web page, sentence, TITLE or META tag.

Keyword Proximity

Keyword proximity refers to the closeness between two or more keywords. In general, the closer the keywords are, the better.

For example:

How Keyword Density Affects Search Engine Rankings

How Keyword Density Affects Rankings In Search Engine

Using the example above, if someone searched for "search engine rankings," a web page containing the first sentence is more likely to rank higher than the second.

The reason is because the keywords are placed closer together. This is assuming that everything else is equal, of course.