A change in the direction of a traveling particle, usually without loss of particle kinetic energy, representing a change in velocity without a change in the scalar speed of the particle. 2. A change in the direction of a wave, beam, electron, or other entity, such as might be accomplished by an electric or magnetic field. Note: If the deflection is caused by a prism (refraction,) a mirror (reflection,) or optical grating (diffraction,) the specific applicable term should be used.