The general property of directional discrimination displayed by systems that receive or emit waves. Thus, loudspeakers, microphones, radio antennas, underwater sound projectors, hydrophones, and even telescopes all have the common property that their effectiveness depends upon the direction from which the wave is either emitted or received. The manner in which a sender or receiver is directional depends largely upon its geometrical shape and, in particular, upon its dimensions compared to those of the wavelength involved (λ).