Lithosphere
The lithosphere is Earth's rigid outer shell, consisting of the crust and the uppermost mantle, broken into the tectonic plates.
Definition
The lithosphere is the hard, rocky outer shell of Earth, including the crust and the stiff top part of the mantle, and it is cracked into big pieces called tectonic plates. It averages about $100\ \text{km}$ thick, thinner under oceans and thicker under old continents, and the plates ride on the softer asthenosphere beneath; the layer is defined by mechanical behavior (rigid), not by composition. In physical terms, the lithosphere is the thermal and mechanical boundary layer of mantle convection, defined by the depth at which temperature reaches roughly $1{,}300^\circ C$ and rock begins to deform by ductile creep. Oceanic lithosphere thickens with age as it cools, roughly as the square root of age, from near zero at the ridge to about $100\ \text{km}$ at $80$ million years, and its increasing density is what drives subduction. Continental lithosphere beneath cratons can exceed $200\ \text{km}$ and is chemically depleted and buoyant, which is why it has survived for billions of years.
Example
The lithosphere is like the cracked shell of a hard-boiled egg, where each piece of shell is a plate that can slide around a little. The North American Plate, which carries Charlotte and everything from the Mid-Atlantic Ridge to the San Andreas Fault, is a single piece of lithosphere that includes both continental and oceanic crust. Flexural bending of the oceanic lithosphere under the load of the Hawaiian volcanoes creates a moat and outer rise around the islands, allowing its effective elastic thickness (about $30\ \text{km}$) to be measured. Seismic tomography images the lithosphere-asthenosphere boundary as a sharp drop in shear-wave velocity.
Key Insight
"Litho" means rock in Greek, so the lithosphere is the rock sphere, the part of Earth that is stiff enough to break. The lithosphere and the crust are not the same thing: the crust is the top layer of the lithosphere, the boundary between them (the Moho) is a change in rock type, and the base of the lithosphere is a change in strength. Plate tectonics works because the lithosphere is strong enough to transmit stress across thousands of kilometers but weak enough to break at its edges. A planet with a lithosphere that never breaks, such as Venus appears to be today, has no plate tectonics.