What is the definition or description of: smooth muscle?

See below: Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit (unitary) and multiunit smooth muscle. Smooth muscle is found within the walls of blood vessels. Smooth muscle is also found in lymphatic vessels, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, arrector pili of skin, and other places.