From AMS Glossary
(Also called radiation pattern, beam pattern, lobe pattern.) A graphical representation of the radiating properties of an antenna as a function of space coordinates.
Typically, the antenna pattern is a diagram showing the radiant intensity, the electric field strength, or the radiant power density emitted from an antenna as a function of the direction from the antenna, in the far-field region unless specified otherwise. The direction of maximum intensity defines the main lobe of the antenna pattern. Other local maxima, weaker than the main lobe, define the secondary lobes or sidelobes of the antenna. In the main lobe, the angular separation in a given plane between the directions at which the intensity falls to one-half its maximum value defines the half-power or 3-dB beamwidth in the plane. The directional sensitivity of an antenna to incoming signals is described by a function identical to the antenna pattern for transmission. Antenna patterns are also used to depict properties of the emitted electromagnetic field other than intensity, for example, the polarization as a function of position in the main lobe.