Half-power points

From Glossary of Meteorology

half-power points

The points on the radiation pattern of an antenna where the transmitted power is one-half that of the maximum, both measured at the same range.

The maximum usually lies along the axis of the beam, and therefore the angle subtended at the antenna by the half-power points is used to define a beam width. The beam width need not be the same for all planes through the axis of the beam.