It means advantage which means the player is one point away from winning the game, set or match
Advantage
ad = advantage
the server
Women's Tennis Association
International Tennis Federation.
ad
The ad side of a tennis court is the players left side. Or the opponents right side.
Association of Tennis Professionals
Usually on the tennis court.
It is either ad in or ad out. If the server won the point it is ad in and if the returner won the point then it is ad out.
it stand for library
Association of Tennis Professional .