A vessel in which the ingredients of mixed drinks are combined with ice and shaken violently.
Glass-bodied shakers are best -- metal ones tend to contract with the cold and get stuck closed.
Look for a shaker with a built-in strainer in the top instead of one with a tiny spout. If your drink calls for fresh-squeezed citrus (and if it doesn't, it should!) the pulp will clog a tiny spout.