1. A crush a girl gets on another girl. The smash can be platonic, meaning that the girl wants to be friends with the girl she has a smash on.
2. The girl another girl has a crush on.
(I noticed this word in a Smith College yearbook from the 50's or 60's, so I think it started with the platonic sense.)
I've got a smash on that girl jane in my drawing class! She has the best clothes.