Antenna beamwidth is the angular distance between:

Antenna beamwidth is standardly defined as the angular distance between the two points on the main (major) lobe where the radiated power drops to one-half of its maximum value. These points are also known as the half-power points or the -3 dB points. Distractors often try to confuse you with the -6 dB points, which would represent one-quarter power, or they refer to minor lobes instead of the major lobe. Always remember that "half-power" and "-3 dB" are interchangeable terms when measuring antenna directivity and defining the edges of the main beam.