定义：
在给定束腰半径的情况下，光束发散角达到可以得到的最小值的光束。

在给定波长情况下，如果激光光束能够聚焦成非常小的斑点，也就是其光束质量是理想的，就称该激光光束是衍射极限的。更准确的说，在束腰半径一定时，经过曲面镜聚焦后的光束发散角是最小的。

其准确含义与光束半径和光束发散角的定义也有关系。如果二者定义中采用了二阶矩，高斯光束能得到光束参量乘积的最小值，这是强度分布为高斯型并且束腰处的相位波前是平的。 

工作于单个横向的谐振腔模式的激光器可以得到衍射极限的输出，因为单模通常指谐振腔的基模，基模的形状一般接近于高斯型。增益介质中的光束失真会使基模变形或者造成多模振荡，从而使光束质量变差。

激光器的光束质量不仅与腔内失真强度有关，还与谐振腔设计细节有关系；优化谐振腔设计对得到衍射极限的输出至关重要。 给定光功率的情况下，衍射极限的光束具有最高的亮度。

Definition: beams with a minimum possible beam divergence for a given waist radius

A laser beam is called diffraction-limited if its potential to be focused to small spots is as high as possible for the given wavelength, limited only by the unavoidable diffraction. In other words, its beam quality is ideal.

More precisely, ideal beam quality means that a beam waist with a given beam radius, generated from the beam by focusing e.g. with a curved mirror, is associated with the minimum possible beam divergence angle. What exactly this means depends on the definition of the beam radius and beam divergence. If second moments are used for the definition of both quantities, the minimum beam parameter product is reached for a Gaussian beam, which has not only a Gaussian intensity profile but also flat wavefronts at the beam waist (beam focus).

A laser which operates on a single transverse resonator mode will usually have a diffraction-limited output, since that single mode is usually the fundamental resonator mode, and the shape of this is usually close to Gaussian. Non-ideal beam quality can be caused by intracavity beam distortions, e.g. in the gain medium, which can either deform the fundamental mode or (more frequently) cause transverse multimode operation. Note that the beam quality of a laser depends not only on the strength of intracavity distortions, but also on certain details of the resonator design; optimization of the latter can be essential for reaching a diffraction-limited output.

For a given optical power, a diffraction-limited beam has the highest brightness.