involuntary muscle

(physiology) Muscle not under the control of the will; usually consists of smooth muscle tissue and lies within a vescus, or is associated with skin.

Muscle, such as smooth muscle of the gut and cardiac muscle, not usually under voluntary, conscious control.

Muscle, such as smooth muscle and cardiac muscle, which is not normally under voluntary, conscious control. However, the term is a misnomer because many so-called ‘involuntary’ muscles (e.g. the anal sphincter muscle) can be brought under conscious control by training.

Any of the smooth muscles, except for the cardiac muscle, not under control of the will.

The noun has one meaning:

Meaning #1: a muscle that contracts without conscious control and found in walls of internal organs such as stomach and intestine and bladder and blood vessels (excluding the heart)
  Synonym: smooth muscle