1. 顕微鏡の下で筋状に見えない筋肉組織(muscle tissue that does not appear striated under the microscope)
2. 意識的な制御なしに収縮する、胃、腸、膀胱、血管などの(心臓を除く)内臓器官の壁に見られる筋肉(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))