1.NounA flat muscle situated under the external oblique muscle in the lateral and ventral part of the abdominal wall, that have aponeuroses extending medially to ensheathe the rectus muscles and fusing in the midventral line in the linea alba, and that act to compress the abdominal contents and to assist in expelling the contents of various visceral organs (as in urination, defecation, parturition, and expiration).