Primitive equations

From Glossary of Meteorology

primitive equations

The Eulerian equations of motion of a fluid in which the primary dependent variables are the fluid's velocity components.

These equations govern a wide variety of fluid motions and form the basis of most hydrodynamical analysis. In meteorology, these equations are frequently specialized to apply directly to the cyclonic-scale motions by the introduction of the so-called filtering approximations.
See equations of motion.