I. ˈdel-ˌtȯid noun
Etymology: New Latin deltoides, from Greek deltoeidēs shaped like a delta, from delta
Date: circa 1681
: a large triangular muscle that covers the shoulder joint and serves to raise the arm laterally
II. adjective
Date: circa 1753
1. : having a triangular shape
a deltoid leaf
— see leaf illustration
2. : relating to, associated with, or supplying the deltoid