A waveguide is a structure that guides waves by restricting the
transmission of energy to one direction. Common types of waveguides include
acoustic waveguides which direct sound, optical waveguides which direct light,
and radio-frequency waveguides which direct electromagnetic waves other than
light like radio waves.
Without the physical constraint of a waveguide, waves would expand into three-
dimensional space and their intensities would decrease according to the
inverse square law.
There are different types of waveguides for different types of waves. The
original and most common meaning is a hollow conductive metal pipe used to
carry high frequency radio waves, particularly microwaves.