1. In radio astronomy, the ratio between the power delivered to the receiver by an antenna and the flux density of the incoming radiation; also known as effective area. The effective aperture of a parabolic antenna is generally smaller than its geometric aperture, because of imperfections in the reflecting surface and the inefficiency of the feed in collecting radio waves reflected from all parts of the dish. An effective aperture can be defined even if no geometric aperture exists, as for example in an interferometer.

2. The diameter of a single mirror that would have the same light-gathering power as a telescope mirror composed of several smaller individual reflectors.