opponens pollicis

noun

opponens pol·​li·​cis -ˈpäl-ə-səs 

: a small triangular muscle of the hand that is located below the abductor pollicis brevis, arises from the trapezium and the flexor retinaculum of the hand, is inserted along the radial side of the metacarpal of the thumb, and functions to abduct, flex, and rotate the metacarpal of the thumb in opposing the thumb and fingers