PageRank - numeric value assigned by Google on a website to measure its importance; in general, the higher the PageRank, the more important a website is

Keyword proximity - refers to the space or closeness between keywords; the closer, the better

Keyword density - the number of times a keyword or key phrase appears on a web page, as compared to the number of words present on that page

Keyword stuffing - a black hat SEO technique in which a page contains too many keywords and key phrases

Bad neighborhood - a website that is composed of unrelated sites; link farm is a typical example; linking to bad neighborhoods will be penalized by search engines