From a woodworking persepctive, a mortise is a hole that is made to receive a tenon (i.e. projecting piece of wood) so as to form a joint and fasten the components together. With this imagery in mind, the ankle mortise is the joint space between the superior tibia and fibula bones and the inferior talus bone, where the "tenon" is the talus and the "hole" is the subtle concavity of the distal tibia and fibula.