Earth's rigid outer shell, made of the crust and upper mantle and broken into tectonic plates, is called the lithosphere.
The word comes from the Greek lithos, meaning “stone,” and sphaira, meaning “sphere.” The lithosphere includes Earth's crust plus the stiff, uppermost part of the mantle. It is not one unbroken shell: it is divided into tectonic plates that move slowly over the weaker, hotter asthenosphere beneath them.
The lithosphere behaves rigidly over short time scales, but it can bend, fracture, and participate in enormous geological movements over millions of years. Plate interactions create earthquakes, volcanoes, mountain ranges, ocean trenches, and new seafloor. Oceanic lithosphere is generally thinner and denser than continental lithosphere, so it can sink beneath another plate at a subduction zone.
A common mix-up is confusing the lithosphere with the asthenosphere. The asthenosphere is part of the upper mantle below the lithosphere and is sufficiently hot and weak to flow slowly. The lithosphere also differs from the crust: the crust is only its outer chemical layer, while the lithosphere includes rigid upper mantle as well.