Crush
A crush is when you develop feelings toward another human, wether it be a boy or girl. You try to impress him/her, make him/her want to date you. usually it doesn't work like that, and you get rejected.

The reason they call it a crush is because your self confidence and will to live get crushed when they say "no I don't like you back"
Dude I think I have a crush on her!

Go talk to her, but don't get your hopes up, she might not like you back.