The layer in the mantle between the lithosphere and the upper mantle is called the asthenosphere.

Here's a breakdown:

* Lithosphere: The rigid outermost layer of Earth, composed of the crust and the uppermost part of the mantle.

* Asthenosphere: A soft, partially molten layer of the upper mantle. It's characterized by its relatively low viscosity, allowing for slow but significant movement within the mantle.

* Upper mantle: The layer of the mantle that extends from the base of the lithosphere to the top of the lower mantle.

The asthenosphere is crucial for plate tectonics. The movement of the tectonic plates is driven by convection currents within the asthenosphere.