Anterior cruciate ligament: A ligament in the knee that crosses from the underside of the femur (the thigh bone) to the top of the tibia (the bigger bone in the lower leg). Abbreviated ACL. Injuries to the ACL can occur in a number of situations, including sports, and can be quite serious, requiring surgery.
The anterior cruciate ligaments, one on either side of the knee, are so called because they cross each other in front of the knee. "Cruciate" taken from the Latin "crux" for "cross" means "in the form of a cross." See: Anterior cruciate injury.