A "pec" is an informal term for the pectoral muscle, which is a muscle connecting the ventral walls of the chest with the bones of the upper arm and shoulder. The pectoral muscle is divided into two parts: the pectoralis major and the pectoralis minor. The pectoralis major is a thick, fan-shaped muscle that lies underneath the breast and serves to flex, extend, and rotate the humerus, the long bone of the upper arm. The pectoralis minor is a thin, triangular muscle located beneath the pectoralis major and attaches to the ribs, serving to stabilize the scapula, the large bone of the shoulder.

In addition to its anatomical meaning, "PEC" can also refer to a Physicians Emergency Certificate, which is a legal document that permits a physician to detain a person against their will for up to 72 hours if they are determined to legally need treatment for mental illness or substance abuse and are unwilling to do a voluntary admission.