- A slender threadlike object, or a section of material so shaped. For example, a conducting thread, or a fiber of glass.
- A thin filament of a transparent material, such as glass or plastic, which is capable of transmitting light signals through successive internal reflections. In order for a fiber to guide light, the proper relationship between the refractive index of the core and its surrounding cladding must be maintained. Also called optical fiber, or light guide.