1. A reference to the zone beneath the earth's surface that lies beneath the lithosphere and consists of several hundred kilometers of weak material that readily yields to persistent stresses: According to the theory of plate tectonics, the Earth's lithosphere is a rigid outer layer that moves slowly over the weaker asthenospheric zone.
2. Etymology: from Greek astheneia, "weakness"; which consists of a-, "without" + sthenos, "strength" + -spheric, "being around" or "a zone".