A waveguide is a structure that guides waves, such as electromagnetic waves or sound waves. They enable a signal to propagate with minimal loss of energy by restricting expansion to one dimension or two. This is a similar effect to waves of water constrained within a canal, or why guns have barrels that restrict hot gas expansion to maximize energy transfer to their bullets. Without the physical constraint of a waveguide, signals will typically dissipate according to the inverse square law as they expand into three dimensional space.