delta, delt-; Δ, δ

(Greek: triangular; fourth letter; Δ, δ, of the Greek alphabet)

deltopectoral (adjective), more deltopectoral, most deltopectoral
Of or relating to the pectoral muscles and the deltoid muscles.
deltopectoral triangle (s) (noun), deltopectoral triangles (pl)
An anatomic triangle formed by the clavicle (bone extending from the breastbone, sternum, at the base of the front of the neck to the shoulder), the deltoid, and the pectoralis major (a skeletal muscle that rotates the arm): The deltopectoral triangle or "clavipectoral triangle" contains the cephalic vein which is a large vein of the arm that empties blood into the axillary (armpit) vein.
subdeltoid (adjective), more subdeltoid, most subdeltoid
Under the large muscle that stretches from the clavicle (collarbone) to the humerus (the long bone in the upper arm) and so covers the shoulder: The subdeltoid bursa is a fibrous sac or cavity that is lined with synovial membranes and filled with fluid that is between the parts that move on each other so the bones don't rub directly against each of the connected joints.
subdeltoidal (adjective), more subdeltoidal, most subdeltoidal
Beneath the triangular muscle of the shoulder area that forms the rounded outer part of the upper arm: The subdeltoidal region is under the scapula (shoulder blade) and the clavicle (collarbone) and can be a cause of pain of certain tendons and joints of the bones; especially, during movements of the joints.

You can see all of the letters of the Greek Alphabet on this page.