A
gathering of
people with similar
interests or
goals. I've been to
science fiction/
fantasy conventions and
cognitive science conventions and I have to say that the
former are a lot more
fun.
At cognitive science conventions, people give talks about cognition all day long and then mingle with one another at night to try to catch up with colleagues and network. I suspect that there is also a lot of trying to get laid going on, but it's all beneath the surface.
At science fiction/fantasy conventions, some people hold panel discussions about various topics related (sometimes only tangentially) to SF, some people watch anime, and some people just hang out in the con suite. There's usually a masquerade and a dance and room parties. The attempts to get laid are much more overt. Men tend to outnumber women substantially which can be a pain in the ass for the women.
There are lots of other kinds of conventions too, I just haven't been to any of them.