Keyword density
The key phrases that you've chosen to target should feature within the visible text content of the pages you are optimising. The proportion of times your phrase is featured compared to the number of words on the page is known as "keyword density".
In the early days of the search engines, keyword density was one of their primary means of determining relevancy. This led to the practice of "keyword stuffing", whereby the key phrases were repeated over and over to ensure the keyword density was higher than 10%.
More recently, a keyword density of 1.5% - 4% was considered to be the optimum level, as the search engines got more sophisticated and tried to eliminate "spam" sites from their indexes. (Spam being the kind of sites that only appear listed highly as a result of such bad practice techniques as "keyword stuffing).
The issue has become even more complicated since then, though, as the major search engines - particularly Google - attempt to improve their SERPs through injecting some understanding into their keyword density methodology, rather than simply counting the number of times certain keywords are featured on a page.
The idea here is that the search engines will look for "semantic variations" of words - i.e. "museum" and "museums", and also synonyms and related words, such as "gallery", "collections", "exhibitions" and the like.
Thus if you wished to promote your site for the phrase
museum
you should ensure your visible text content featured those words, as well as similar ones such as those above, in real sentences.