smooth muscle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy Involuntary muscle which is found within the intestines, throat, uterus, and blood vessel walls.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun muscle tissue that does not appear striated under the microscope; has the form of thin layers or sheets
• noun 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)