A knee ligament is a band of fibrous tissue that connects two bones in the knee joint, providing stability and support. There are four main knee ligaments - the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL) and lateral collateral ligament (LCL). These ligaments work together to prevent excessive movement of the lower leg bone in relation to the thigh bone, allowing for smooth joint motion.