在物理学里，相干性（coherence）指的是，为了产生显著的干涉现象，波所需具备的性质。更广义地说，相干性描述波与自己、波与其它波之间对于某种内秉物理量的相关性质。

当两个波彼此相互干涉时，因为相位的差异，会造成建设性干涉或摧毁性干涉。假若两个正弦波的相位差为常数，则这两个波的频率必定相同，称这两个波「完全相干」。两个「完全不相干」的波，例如白炽灯或太阳所发射出的光波，由于产生的干涉图样不稳定，无法被明显地观察到。在这两种极端之间，存在着「部分相干」的波。

相干性又大致分类为时间相干性与空间相干性。时间相干性与波的带宽有关；而空间相干性则与波源的有限尺寸有关。

In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency. It is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference. It contains several distinct concepts, which are limiting cases that never quite occur in reality but allow an understanding of the physics of waves, and has become a very important concept in quantum physics. More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets.