sternocleidomastoid

(stûr'nō-klī'də-măs'toid)
n.
Either of two muscles of the neck that serve to flex and rotate the head.

[STERN(UM) + Greek kleis, kleid-, collarbone + MASTOID.]

n

A muscle of the neck that is attached to the mastoid process and superior nuchal line and by separate heads to the sternum and clavicle. It functions with other muscles to turn the head from side to side and tilt the head to one side or the other.

sternomastoid

A two-headed muscle deep on the anterolateral surface of the neck. The muscle has one of its origins on the manubrium of the sternum and the other on the medial aspect of the clavicle; its insertion is on the mastoid process (a nipple-like extension) of the temporal bone. The primary actions of the sternocleidomastoid are flexion of the neck, extension of the head, and lateral flexion about the spine. It also acts as an accessory inspiratory muscle, elevating the sternum.

(stûr'nō-klī'də-măs'toid)
adj.

Relating to or characterizing the sternum, the clavicle, and the mastoid process.

n.

The sternocleidomastoid muscle.

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The noun has one meaning:

Meaning #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
  Synonyms: sternocleidomastoid muscle, sternocleido mastoideus, musculus sternocleidomastoideus

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