A triple-double is when you record three double-digit stats. The most common triple-double involves points, assists, and rebounds. For example, 24 points, 12 assists, and 14 rebounds. You can have a triple-double involving any stat except for turnovers. A player could only score 4 points and get only 3 assists, but get a triple-double by having 13 rebounds, 11 blocks, and 15 steals. That is a rarity, but it counts as a triple-double.
To be more precise, it is when a player records double-digit values for three statistical categories. It is not limited to points, rebounds, and assists although this is the typical method by which players attain a triple-double
A Basketball player accomplishes a triple double by reaching double digits in 3 of the following: assists, rebounds, points, steals, and blocks. In most cases the 3 doubles are assists, rebounds, and points.
There have also been 4 quadruple doubles in the NBA, when players reached double digits in 4 of the above stats. The quadruple double has only been possible since the 1973-1974 season, when the NBA began counting steals and blocks.