Asthenosphere

the region below the lithosphere, variously estimated as being from fifty to several hundred miles (eighty-five to several hundred kilometers) thick, in which the rock is less rigid than that above and below but rigid enough to transmit transverse seismic waves.
noun
a thin semifluid layer of the earth (100–200 km thick), below the outer rigid lithosphere, forming part of the mantle and thought to be able to flow vertically and horizontally, enabling sections of lithosphere to subside, rise, and undergo lateral movement see also isostasy
n.

layer of the earth’s upper mantle, 1914, from greek asthenos (see asthenia) + sphere.
asthenosphere
(ās-thěn’ə-sfîr’)
the upper part of the earth’s mantle, extending from a depth of about 75 km (46.5 mi) to about 200 km (124 mi). the asthenosphere lies beneath the lithosphere and consists of partially molten rock. seismic waves p-ssing through this layer are significantly slowed. isostatic adjustments (the depression or uplift of continents by buoyancy) take place in the asthenosphere, and magma is believed to be generated there. compare atmosphere, hydrosphere, lithosphere.