A combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture.

Synonym of Lombard effect

plural of refreshment car

(rail transport) A vehicle included in a train, in which refreshments (food and drink) are provided to passengers.

an approach to mathematics education, particularly in North America

Synonym of jog someone's memory.

Synonym of multi-level marketing

plural of reference frame

(physics) Alternative form of frame of reference

A reflector with a curved reflecting surface that forms part of a paraboloid (as opposed to, for example, a sphere)