Keyword prominence explain the situation of the keyword appear on a webpage. It is normally seen that the appearance of a keyword in the start or the top of a page or the title or the Meta tag heightens the prominence or relevance.It is equally important to keyword density and frequency of usage. CMIIW
In simple term it is the density of keyword in the web page content or you can say that how many times it is used in content.
Search engine optimization, this refers to the prominent placement of keywords or phrases within a Web page. Prominent placement may be in the page header, meta tags, opening paragraph, or start of a sentence.