eight ball

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A black pool ball that bears the number eight.
• noun A game in billiards played with a cue ball and 15 object balls in which the winner must pocket the eight ball after pocketing the seven balls numbered either higher than or lower than the eight ball, with the requirement that the cue ball must first strike one of the seven balls allotted to a player for a shot to be legitimate. A player who pockets the eight ball before the other seven automatically loses.
• idiom (behind the eight ball) In an unfavorable or uncomfortable position.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun billiards, snooker, pool Alternative form of eight-ball.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a black pool ball bearing the number 8; should be the last to go in certain pool games