What is charley horse, the condition related to leg pain?

A 'charley horse' is a common term for a sudden, painful, involuntary muscle cramp, usually in the calf, foot, or thigh, according to the Cleveland Clinic. It causes involuntary muscle tightening that can last seconds to minutes. Such cramps may occur due to involuntary nerve discharges, poor blood flow, stress or too much high-intensity exercise.