ec2-3-236-204-251.compute-1.amazonaws.com | ToothyWiki | RecentChanges | Login | Webcomic Japanese. Usually means "lewd", "suggestive", "sexual", or something of the sort. Literally is the letter "H", the first letter of "hentai", which means "pervert" or "perverted".
A person, as an adjective or a general maddened cry of rage... often said to a male anime character, by a female one to whom he's just said or done something suggestive.
An anime (or other piece of fiction or entertainment) to indicate it contains scenes of this kind.
A camera angle, a facial expression, just about anything.
[This page] has a couple of entertaining rants about the difference between "ecchi" (or "H") and "hentai", and the ways they're often used incorrectly by Western fans.
Update: Not any more it doesn't - it just has a brief summary instead.