定义：衍射被波导结构抑制的波。

导波是在波导结构（一种空间非均匀结构，可以阻碍衍射引起的光束扩展）中传播的光波。
波导结构可以有效消除导波的发散性，还会改变其它性质，例如波数和色散（参阅波导色散）。
因此，导波与平面波在很多方面都有不同的特性。这些变化当波导结构并不比波长大很多时尤其大。 

一种很重要的导波的例子为在光纤中传播的光波

Definition: waves for which diffraction is suppressed by a waveguide structure

Guided light waves are waves propagating in a waveguide structure, i.e., a spatially inhomogeneous structure which counteracts the beam expansion that would normally be caused by diffraction. The waveguide structure effectively eliminates the divergence of a guided wave, but also modifies other properties such as its wavenumber and chromatic dispersion (→ waveguide dispersion). Therefore, the properties of guided waves deviate in various respects from those of, e.g., plane waves. Such deviations are particularly large in cases where the waveguide dimensions are not much larger than the wavelength.

A particularly important example of guided optical waves is light propagating in optical fibers. These are flexible waveguides, i.e., they can be substantially bent before significant bend losses arise.