what is a WASP?

I've come across this term before and never given it a second thought.

WASP

Is it an acronym?
What does it mean?

Here's a passage from the story I'm currently reading:-

"She was a sophisticated New Yorker and a brilliant young lawyer with no place to go but up.
There were two things she wasn't though.
She wasn't an international beauty and she wasn't a WASP".

Can anybody out there enlighten me?