One of standard elements of web page optimization is Keyword Density: up until very recently ratio of keywords to rest of body text was generally deemed to be one of most important factors employed by search engines to determine a web site's ranking.
However, this basically linear approach is gradually changing now: as mathematical linguistics and automatic content recognition technology progresses, major search engines are shifting their focus towards "theme" biased algorithms that do not rely on analysis of individual web pages anymore but, rather, will evaluate whole web sites to determine their topical focus or "theme" and its relevance in relation to users' search requests.
This is not to say that keyword density is losing in importance, quite contrary. However, it is turning into a lot more complex technology than a simple computation of word frequency per web page can handle.
Context analysis is now being determined by a number of auxiliary linguistic disciplines and technology, for example: * semantic text analysis * textlexical database technology * distribution analysis of lexical components (such as nouns, adjectives, verbs) * evaluation of distance between semantic elements * AI and data mining technology based pattern recognition; * term vector database technology etc.
All these are now contributing to increasing sophistication of relevance determination process. If you feel this is beginning to sound too much like rocket science for comfort, you may not be very far from truth: it seems that future of search engine optimization will be determined by what industry is fond to term "word gurus".
A sound knowledge of fundamental linguist methodology plus more than a mere smattering of statistical calculus will most probably be paramount to achieve successful search engine rankings in foreseeable future. Merely repeating well worn mantra "content is king!", as some of lesser qualified SEO professionals and very many amateurs are currently doing, may admittedly have a welcome sedative effect by creating a feeling of fuzzy warmth and comfort. But to all practical purposes it is tantamount to whistling in dark and fails miserably in doing justice to overall complexity of process involved.
It should be noted that we are talking presence AND future here: many of classical techniques of search engine optimization are still working more or less successfully, but there is little doubt that they are rapidly losing their cutting edge and will probably be as obsolete in a few months' time as spamdexing or invisible text - both optimization techniques well worth their while throughout 90s - have become today.
So where does keyword density come into this equation? And how is it determined anyway?
There's rub: term "keyword density" is by no means as objective and clear-cut as many people (some SEO experts included) will have it! The reason for this is inherent structure of hypertext markup language (HTM) code: as text content elements are embedded in clear text command tags governing display and layout, it is not easy to determine what should or should not be factored into any keyword density calculus.
The matter is complicated further by fact that meta tags inside a HTML page's header may contain keywords and description content: should these be added to total word count or not? Seeing that some search engines will ignore meta tags altogether (e.g. Lycos, Excite and Fast/Alltheweb), whereas others are still considering them (at least partially), it gets even more confusing. What may qualify for a keyword density of 2% under one frame of reference (e.g. including meta tags, graphics ALT tags, comment tags, etc.) may easily be reduced to 1% or less under another.