A squab is a young domestic pigeon, the domesticated version of the wild rock pigeon. Most people use the word squab when they're talking about this bird as food.
If you order squab in a restaurant, you'll be served what looks like a small roasted chicken. The word squab was once used for the meat of any dove or pigeon—including the now-extinct passenger pigeon. In the 17th century, it simply meant "very young bird," but earlier it was used to mean "unformed, lumpish person" or "lumpish mass." The origins of squab are uncertain, but it's probably related to the Swedish skvabb, meaning "loose, fat flesh."