a liking to a specific female particular whose physical and emotional characteristics you admire while being heterosexual and an actual female yourself.
Note: this is very much so on the brink of bisexuality or homosexuality and could lead to girl on girl experimentation.
Girl: "Crikey,look at her! She's so nice and pretty and cute. ::sigh::"
Dude: "You totally have a ladycrush on her, why don't you invite her over to your house and we could all watch a movie on the couch?"
Girl: "That sounds amazing"
Dude: "Yes it does, girl, yes it does...."
A crush one girl can have on another generally famous female without being cast as bisexual or gay.
Guy: Katy Perry is so hot.
Girl: I know! She's totally my lady crush.