In telecommunication, a planar array is an antenna in which all of the elements, both active and parasitic, are in one plane.

Note 1: A planar array provides a large aperture and may be used for directional beam control by varying the relative phase of each element.

Note 2: A planar array may be used with a reflecting screen behind the active plane.

Source: from Federal Standard 1037C and from MIL-STD-188