/sterr'noh kluy'deuh mas"toyd/ , Anat.
adj.
1. of, pertaining to, or involving the sternum, the clavicle, and the mastoid process.
n.
2. a thick muscle on each side of the neck, the action of which assists in bending the head and neck forward and sideways.
[ 1820-30; STERNO- + Gk kleido-, comb. form of kleís key, clavicle + MASTOID ]