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{{#category_index:R|retrograde}} (re’ tr∂ grād,) 1. Rotational motion opposite to the usual direction. Rayleigh waves are sometimes called retrograde waves because motion near the surface is in elliptical orbits such that the particle is traveling opposite to the direction of the wave while at the top of the ellipse; see Figure R-3. 2. A branch of a time-distance curve resulting from very steep velocity gradients. See Figure D-19b. Also called reverse branch (q.v.).