latissimus dorsi

n., pl., la·tis·si·mi dorsi (-mī').
Either of two broad, flat, triangular muscles running from the vertebral column to the humerus.

[New Latin lātissimus dorsī : Latin lātissimus, superlative of lātus, wide + Latin dorsī, genitive of dorsum, back.]

lats

A broad, flat triangular muscle occurring on both sides of the lower back, covered superiorly by the trapezius and contributing to the posterior wall of the axilla (armpit). This is the broad back muscle, which swings the arm backwards and rotates it inwards. It has its origins on the lower six thoracic vertebrae and all lumbar vertebra, the posterior sacrum, the iliac crest, and the lower three or four ribs. Insertions are on the inter-tubercular groove of the femur. The lats primary actions about the shoulders are extension, adduction, and medial rotation. The lats play an important part in bringing the arm down in a power stroke, as in striking a blow, swimming the front crawl, and rowing.

(lă-tĭs'ə-məs dôr'sī)
n.

A muscle with origin from the spinous processes of the lower thoracic and lumbar vertebrae, the median ridge of the sacrum, and the outer lip of the iliac crest, with insertion into the humerus, with nerve supply from the thoracodorsal nerve, and whose action adducts the arm, rotates it medially, and extends it.