Which is to say, a young pike, or any of a couple smaller species of freshwater predatory fish of the genus Esox, closely related to the pike. The young pike meaning is largely British usage, and the original meaning. In Canada, the name is also applied to the walleye (Sander vitreus). Note that like fish itself, the plural is identical unless you are talking about more than one type of pickerel, in which case it's pickerels. The word itself is old, being a 13th century diminutive of pike, named for its snout after the sharply pointed tool (pīc in Old Enlgish) + the pejorative French suffix -rel, which suggests the word was first coined in Anglo-Norman.
The common denominator, wherever you find pickerel, is shallow, weedy water.