So let's begin with some definitions for the word syndrome.
Definition of SYNDROME
1. : a group of signs and symptoms that occur together and characterize a particular abnormality or condition
2. : a set of concurrent things (as emotions or actions) that usually form an identifiable pattern
Also, from http://medical-dictionary.thefreedictionary.com/syndrome
Syndrome /syn·drome/ (sin´drōm) a set of symptoms occurring together; the sum of signs of any morbid state; a symptom complex.
Now, let's move on to definitions for thoracic outlet.
The cavity that exists between your first rib and your clavicle is filled with nerves, veins, and muscle When any of them become compressed, a condition called thoracic outlet syndrome (TOS) can develop The cavity that exists between your first rib and your clavicle is filled with nerves, veins, and muscle. A Quick Look at Thoracic Outlet Syndrome by Big Oak, Inc.
So now we put it all together.
Thoracic Outlet Syndrome- any of several neurovascular syndromes due to compression of the brachial plexus nerve trunks, with pain, paresthesias, vasomotor symptoms, and weakness and small muscle wasting in upper limbs; causes include drooping shoulder girdle, a cervical rib or fibrous band, an abnormal first rib, limb hyperabduction (as during sleep), or compression of the edge of the scalenus anterior muscle.