Cosplay: A Japanese contraction of 'Costume Play', i.e. the act of dressing up in costume.
I've been cosplaying since 1999, and nowadays I'm very involved in helping run conventions, staff and judge masquerades, and teach workshops and classes.
What interests me most about costuming/cosplaying is actually the design and construction of complex elements. I love figuring out how to make things; that's more than half the fun for me. Dressing up for a day or two is fun, but knowing that you figured out something tough and pulled it off really well stays with you for quite a while.