either of the large, wide, triangular muscles, one on each side of the back, connecting the lower and central spine with the arm: it helps rotate, extend, and adduct the arm
See latissimus dorsi in American Heritage Dictionary 4
(lə-tĭsˈə-məs dôrˈsī)
nounpl.la·tis·si·mi dorsi(-mīˌ)
Either of two broad, flat, triangular muscles running from the vertebral column to the humerus.