Help Quick! What does Black Tie Invited mean?

Going to a wedding in a few hours...
DH is saying that he should wear a tux...
I say it's just a fancier way of saying Black Tie Optional.
What do you guys think???

Black tie usually means tux for the guys, cocktail dress or formal gown for the girls.

This is from about.com

A Black Tie invitation calls for formal attire. Men wear tuxedos, women wear cocktail, long dresses or dressy evening separates.