trapezius

n., pl., -us·es.
Either of two large flat triangular muscles running from the base of the occiput to the middle of the back that support and make it possible to raise the head and shoulders.

[New Latin, from Medieval Latin trapēzium, trapezium (from the shape of the muscles paired). See trapezium.]

A large, flat, triangular muscle in the upper back. It originates from the base of the skull and the upper vertebrae and inserts onto the shoulder blade and collar bone. Its main job is to rotate the shoulder blade and move it inwards. It also helps to rotate the head and extend the neck backwards. If you have difficulty holding your arms out to the side or over your head, your trapezius may be weak. Back strengtheners (see back extension) will help to strengthen the trapezius and improve flexibility in the upper back. Overtraining for dance, especially ballet, can tighten the trapezius causing discomfort and restricting movement. Relaxation exercises and physiotherapy may be used to treat the problem, but it is best avoided by not overtraining.

The most superficial muscle of the posterior thorax. The trapezius is flat and triangular in shape with its origins on the occipital bone, the ligamentum nuchae, and the spines of the vertebrae. It has a continuous insertion along the acromion and spine of the scapula, and the lateral part of the clavicle. The trapezius adducts and rotates the scapula upwards (via the upper fibres) and downwards (via the lower fibres). It also assists in turning the head and extending the neck backwards.

Trapezius

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The noun has one meaning:

Meaning #1: either of two flat triangular muscles of the shoulder and upper back that are involved in moving the shoulders and arms
  Synonyms: trapezius muscle, cowl muscle, musculus trapezius

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