beam width

6.69  beam width. At a specified frequency, in a specified plane including the beam axis, the angle included between the two directions, one to the left and the other to the right of the axis, at which the angular deviation loss has a specified value. Unit, degree.

Annotation              Beam widths are commonly specified for an angular deviation loss of 3, 6, or 10 decibels, the choice depending upon the directivity of the transducer or upon its intended application. The particular angular deviation loss may be indicated conveniently by use of a term such as “3-dB beam width.”