The imaginary line that passes through the center of the earth and the north and south poles is the

Earth's axis. More info on the axis from Wikipedia: "The Earth's rotation axis moves with respect to the fixed stars (inertial space); the components of this motion are precession and nutation. It also moves with respect to the Earth's crust; this is called polar motion."

in the analysis of systems that involve fluid flow, we frequently encounter the combination of properties u and pv.