A trick in excess of the number for which you have contracted. Overtricks do not count towards game and, in rubber bridge, you score them above the line.
For example, if you bid 4♠ and make 13 tricks, you have made three overtricks, the difference between the 13 you have made and the 10 you contracted to make.
At matchpoints, making overtricks can be a particularly useful way to improve your score.