optical disk: a disk coated with plastic that can store digital data as tiny pits etched in the surface; is read with a laser that scans the surface
entity: that which is perceived or known or inferred to have its own distinct existence (living or nonliving)
sternocleidomastoid: 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