= Solution
A <waveguide> confines the transverse structure of a wave while permitting propagation along a chosen route. It can deliver <energy> to a distant receiver without the same geometric spreading as an unguided beam, protect a signal from unwanted coupling, and select transverse <normal modes>. It also makes propagation paths and coupling to devices more controllable. Confinement does not eliminate <wave absorption> or scattering loss, and it can introduce <wave dispersion> and <cutoff frequencies of a membrane waveguide> or corresponding cutoffs for other wave types.
Two examples are an <optical fiber>, which guides <light> by its core-and-cladding <refractive index> structure, and a hollow <electromagnetic waveguide> used to carry microwaves between a source and an antenna. A duct for <acoustic waves> is a further example. Each uses different material and <boundary conditions>, but the common idea is controlled transverse confinement.
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