Thoracic outlet syndrome (TOS) consists of a group of distinct disorders (disease) involving compression at the superior thoracic outlet that affect the brachial plexus (nerves that pass into the arms from the neck), and/or the subclavian artery or vein (subclavian vein) (blood vessels that pass between the chest and upper extremity).

The compression may be positional (caused by movement of the clavicle (collarbone) and shoulder girdle (Pectoral girdle) on arm movement) or static (caused by abnormalities or enlargement of the various muscles surrounding the arteries (artery), veins (vein), and brachial plexus).