Waveguide
A waveguide is simply a structure which guides a wave, keeping the energy confined within its geometry. An optical fiber is a waveguide for light, though another example might be a television aerial and the connecting co-axial cable which are waveguides for the radio waves carrying the TV signal into the set.
Related Terms: Core, Double-Clad fiber, Refractive Index, Refractive Index Profile (RIP)