In tennis, the ad court is defined as the left side of the court when divided by the center baseline. This is dependent on each player's orientation to the net. When a player is facing the net, the side of the court on their side of the net on their left is their ad court and vice versa for the other player. The right side of the court is known as the 'deuce' court.
If a player were serving from the ad court, they would be standing to the left of the center mark, behind the baseline. The player would serve across their side diagonally into the other player's ad court. When a player is serving the ball, and the score is an advantage-in or advantage-out, they will always be serving from the ad court, hence the name. Players always begin serving on the deuce side of the court before moving to the ad court. When you are on an odd number of points played, you will be on the ad side. However, if you are at an even number of points played, you will be on the deuce side.