1) A malignant solid tumor arising from mesenchymal tissues which normally differentiate to form striated muscle. It can occur in a wide variety of sites. It is divided into four distinct types: pleomorphic, predominantly in male adults; alveolar (rhabdomyosarcoma, alveolar), mainly in adolescents and young adults; embryonal (rhabdomyosarcoma, embryonal), predominantly in infants and children; and botryoidal, also in young children. It is one of the most frequently occurring soft tissue sarcomas and the most common in children under 15.

2) A malignant tumor of muscle tissue.