A tux is a fancy black suit that you might wear, along with a bow tie, to your senior prom or your wedding.
The word tux is a common North American shorthand for tuxedo. If you're invited to an elegant party that's "black tie only," that means you're expected to wear a long, dressy gown or a tux. Tux was first used in the 1920s. Tuxedo, coined in the late 1800s, was named for a fancy country club called Tuxedo Park, which comes from an Algonquian word — possibly p'tuck-sepo, or "crooked river."