Hamstring: The prominent tendons at the back of the knee. They are the sidewalls of the hollow behind the knee. (This hollow is called the popliteal space). Both hamstrings connect to muscles that flex the knee. A pulled hamstring is a common athletic injury.
The "ham" of "hamstring" comes from an Old Teutonic word "ham" meaning crooked. This is in reference to the crooked part of the leg, that is the knee. To "hamstring" someone is to cripple them.
See Hamstring Injuries and the Hamstring Stretch to prevent hamstring injuries.