Waveguide
Definition
A pipe that channels waves — fiber-optic cables are waveguides for light, and microwave ovens use metal waveguides.
Technical definition
A structure that confines waves by reflection or total internal reflection, supporting only discrete modes above a cutoff frequency. Single-mode fiber carries one clean mode.
Example
Undersea internet cables are bundles of hair-thin glass waveguides, each carrying terabits per second as trapped light.
Common confusion
A waveguide does not amplify the wave — it only prevents it from spreading. The signal still weakens with distance and needs repeaters.