A message board and image board on the Internet where people congregate to post various kinds of pictures, flash animations, and discuss things such as anime. It is the English version of the original Japanese board with a similar system: 2chan.
The system that 4chan uses is Anonymous, so one can safely post one's opinion without worrying. One can use a name and a tripcode (a # followed by a 'password') to gain 'status' on 4chan.
Word entered to the email field of a *chan (iichan, 4chan, 2chan, etc) message board so that the post is not bumped. It is Japanese for 'to lower.' It is the opposite of 'age' (ah-gay), which means to raise.