WordNet (r) 3.0 (2006):

sternocleidomastoid muscle
    n 1: one of two thick muscles running from the sternum and
         clavicle to the mastoid and occipital bone; turns head
         obliquely to the opposite side; when acting together they
         flex the neck and extend the head [syn:
         sternocleidomastoid, sternocleidomastoid muscle,
         sternocleido mastoideus, musculus
         sternocleidomastoideus]