A mode is the energy distribution in which the light travels down the fiber. A multimode fiber can support several modes at once, whilst a single-mode fiber supports only the fundamental mode. Within a fiber the optical field can propagate with peak intensities at various locations. In a single-mode fiber, only the lowest order mode propagates but in multimode fiber, many higher order modes can propagate.
Related Terms: Mode, Mode Field Diameter (MFD), Multimode, Polarization Mode