a contractile type of muscle tissue controlled by the involuntary nervous system, occurring in the walls of the uterus, stomach, intestines, blood vessels, etc., and characterized by nonstriated, spindle-shaped cells
See smooth muscle in American Heritage Dictionary 4
noun
Muscle tissue that contracts without conscious control, having the form of thin layers or sheets made up of spindle-shaped, unstriated cells with single nuclei and found in the walls of the internal organs, such as the stomach, intestine, bladder, and blood vessels, excluding the heart.