光束发散度是指电磁束或光束随着和发射点距离，其束直径或是半径增加的程度，一般会以角度的方式表示。此一词语只在远场下有效，也就是离焦点很远的位置。不过远场也可能很靠近辐射孔，视发射孔孔径及工作波长而定。

光束发散度常用在以光学方式处理电磁束的情形，此时光束的孔径会远大于其波长。不过光束发散度也会用在射频的范围，前提是天线工作在所谓的光学区，特征长度远大于一个波长。

光束发散度常用在光束截面是圆形的情形下，不过也有例外。例如光束截面是楕圆时也可以使用，只是要标明光束发散度参考的位置，例如是楕圆的长轴或是短轴。

The beam divergence of an electromagnetic beam is an angular measure of the increase in beam diameter or radius with distance from the optical aperture or antenna aperture from which the electromagnetic beam emerges. The term is relevant only in the "far field", away from any focus of the beam. Practically speaking, however, the far field can commence physically close to the radiating aperture, depending on aperture diameter and the operating wavelength.