The point after deuce. If the server wins the deuce point, it's 'advantage in' (or ad-in). If the returner wins, it's 'advantage out' (or ad-out). Win the next point from advantage and you win the game. Lose it and you go back to deuce. Ad-out is effectively a break point. The back-and-forth between deuce and advantage is where tennis matches are won and lost.
"Advantage receiver. This is break point. Everything rides on this next point."