A shot hit diagonally from one side of the court to the other. Crosscourt shots travel over the lowest part of the net (the center) and into the longest dimension of the court, giving you the biggest margin for error. This is why crosscourt is the default rally direction. Changing direction from crosscourt to down the line is one of the fundamental tactical decisions in every point.
"Heavy crosscourt forehand rally. Both players working the angles, waiting for someone to go down the line."