Muscle weakness is a common condition that can affect all ages. It refers to the loss of strength in one or more muscles. Some of the synonyms used to describe muscle weakness include asthenia, myasthenia, flaccidity, feebleness, debility, enervation, lassitude, and exhaustion. Asthenia refers to general weakness, while myasthenia specifically relates to muscle weakness caused by a neuromuscular disorder. Flaccidity means limpness or lack of firmness, and feebleness refers to a lack of physical strength. Debility and enervation specifically describe weakness of the body or mind, and lassitude and exhaustion indicate a lack of energy or stamina. Knowing some of these synonyms can help medical professionals better communicate with one another and diagnose and treat muscle weakness more effectively.