Autocomplete (or “autosuggest”) refers to the search suggestions that appear as you type a query into the search box (shown above).
According to Google, these suggested searches are “a reflection of the search activity of users and the content of web pages.” This is a bit vague, but it would be reasonable to assume that if searchers are frequently searching for a term, or if a term is frequently appearing on the web, then that term is more likely to appear in autosuggest.
The search engines have been opaque as to whether clicks on autocomplete terms also factor into their algorithm. Since these clicks initiate a web search, I think there’s sufficient reason to believe they do influence autocomplete suggestions — and if that’s the case, then having terms appear in autocomplete becomes something of a self-perpetuating cycle.