A female performer in a Las Vegas-style show, usually able to dance moderately well (or better), or willing to appear topless before an audience, or sometimes both.
Showgirls, in general (naturally, there are exceptions), tend to look better from a distance than they do up close. Some, like those who appear in Bottoms Up!, a topless comedy act (yes, a very strange combination that somehow actually works), are quite attractive, have nice breasts, and show them to you eagerly. Others, like those who dance in The Rio's Masquerade Show in the Sky, vary between decent and quite hideous, to the point that you're glad you're watching the show from the balcony and can only really see the jiggling breasts and not their faces. A few are actually so bad that you can see the ugliness from a distance, and it still hurts. Sometimes, it's bad enough to break the camera lens.
It seems to me that many women who become showgirls (particularly in the topless shows) either tried out for jobs as strippers and failed, or decided they were "too good" to be strippers and settled for the next worst thing. Granted, there are attractive showgirls, but I've seen plenty of showgirls who shoot out that "I should have been a stripper" vibe, and a few strippers who'd have been better off being a showgirl and not showing it off.