Ordinary ray
From Glossary of Meteorology
ordinary ray
(Or ordinary wave.) When a plane wave is incident on a uniaxial medium (an anisotropic medium with two equivalent orthogonal directions), two transmitted waves result: the ordinary ray (or wave), the phase velocity of which does not depend on its direction, and the extraordinary ray (or wave), the phase velocity of which depends on the angle it makes with the optic axis (the unique direction along which waves propagate as if the medium were optically isotropic).
