Multiple-system atrophy (MSA) is a degenerativeneurological disorder. MSA is associated with the degeneration of nerve cells in specific areas of the brain. This cell degeneration causes problems with movement, balance, and other autonomic functions of the body such as bladder control or blood-pressure regulation. The cause of MSA is unknown and no specific risk factors have been identified. Around 55% of cases occur in men, with typical age of onset in the late 50s to early 60s.

The overall prevalence of MSA is estimated at 4.6 cases per 100,000 people. This disease is more common in men than in women, with studies showing ratios ranging from between 1.4:1 to ratios as high as 1.9:1.

MSA is characterized by a combination of the following, which can be present in any combination:

When autonomic failure predominates, the term Shy-Drager syndrome is sometimes used, although this term is no longer current, given the terminology changes which are explained below.

A variant with combined features of MSA and Lewy body dementia may also exist.

A binary system is an astronomical term referring to two objects in space (usually stars, but also planets, galaxies, or asteroids) which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions (e.g. that of double planet, but not that of binary star) require that this center of mass is not located within the interior of either object. A multiple system is like a binary system but consists of three or more objects.

When binary minor planets are similar in size, they may be called "binary companions" instead of referring to the smaller body as a satellite. Good examples of true binary companions are the 90 Antiope and the 79360 Sila–Nunam systems.