Keyword Density: Keyword density is a measurement used to determine the frequency and prominence of a keyword or keyphrase on a webpage. In the 1990s, during the early days of search engines, keyword density -- and the placement of keywords overall -- was considered a large factor in determining a page's rank on a search engine. Today, however, most broad open search engines, like Google, Yahoo, or Bing, place little to no emphasis on keyword density beyond judging pages for spam content.
In general, however, most experts on the internet consider a density of 1% to 3% to be appropriate for more webpages. In other words, if the keyword for a page is cheese, then the word should appear no more than one to three times per 100 words. However, Google rankings today take into account far more than keyword density, so it's important to not rely solely on this tactic. Where keyword density was a factor in search engine optimization (SEO) campaigns in the past, today it is only part of this practice.
Keyword density may be a factor in more specialized searches on individual databases. Someone searching for an academic dissertation may rely on a keyword search to find more relevant content, so if that person searches for "semiotics," they can find papers that contain more instances of that keyword.