1. In a unidirectional antenna, that portion of the plane surface that is perpendicular to the direction of maximum radiation and through which the major part of the radiation passes. The physical aperture of a horn or parabolic antenna is identical with the actual area of the antenna face. Other types of apertures may be defined that are related to the effectiveness with which an antenna can remove energy from an incident radio wave (usually called effective area), and to the extent to which the intercepted energy is lost in heat or is reradiated. 2. In an opaque disk, the hole or window placed on either side of a lens to control the amount of light passing through.