When it comes to wine, what exactly is a "sleeper"?

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Dear Dr. Vinny,
I used to think I knew what "sleeper" meant, but looking at all the publications labeling everything from good to really crappy wines a "sleeper" I'm beginning to have my doubts. What exactly is a "sleeper"? Something that tastes better after you age it? Something that can sneak up later as a highly rated wine after it comes out of the barrel?
—Jeffrey G., New York
Dear Jeffrey,
A "sleeper" wine is a pleasant surprise, a wine that shows better than expected. It can come from an underappreciated region, variety, vintage or producer, or it could have been unnoticed by critics. In some cases, it could refer to a wine that has aged better than expected.
—Dr. Vinny