a painful experience, very common amoung middle schoolers (and high schooler's and even adults to a lesser degree) that involves being obsessed with a member of the opposite sex (or the same sex, if u prefer), being attracted to them physically (most common), or emotionally- also called 'puppy love'
also: the object of this affection
He has a huge crush on her.
Her crush is hot, but unworthy of her affections.
crushing on him.