Asthenia (Greek: ἀσθένεια, lit lack of strength but also disease) is a medical term referring to a condition in which the body lacks or has lost strength either as a whole or in any of its parts.
Myasthenia (my- from Greek μυο meaning "muscle" + -asthenia ἀσθένεια meaning "weakness"), or simply muscle weakness, is a lack of muscle strength.