Leaky mode: In an optical fiber or other form of waveguide, a mode having a field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance. In order for a leaky mode to be definable as a mode, the relative amplitude of the oscillatory part (the leakage rate) must be sufficiently small that the mode substantially maintains its shape as it decays.

Note: Leaky modes correspond to leaky rays in the terminology of geometric optics. Leaky modes experience attenuation, even if the waveguide is perfect in every respect. Synonym tunneling mode.

Source: from Federal Standard 1037C and from MIL-STD-188