Zonal Flow, Meridional Flow, and Meridian
Definition
On a spherical celestial body, a flow along the east or west direction is called a zonal flow, and a flow along the north or south direction is called a meridional flow. A geodesic curve on the sphere connecting the south pole and the north pole is called a meridian.
Explanation
Although the actual Earth is slightly flattened and is said to be an ellipsoid, when the celestial body is assumed to be a perfect sphere as usual, zonal flows are parallel to the equator, and meridional flows are parallel to meridians. Unlike the equator, of which there is only one, there are infinitely many meridians, and every meridian is perpendicular to the equator.
In fluid dynamics, it is customary to take the direction parallel to the equator as the $x$-axis and the direction parallel to the meridian as the $y$-axis, denoting the zonal flow by $u$ and the meridional flow by $v$.