A wavenumber, wavelength, or frequency band within a Fourier energy spectrum that has a relative minimum of spectral energy. Much of the theoretical development of turbulence in the atmosphere is based on the assumption of a spectral gap between larger-wavelength motions (called mean motions) and small- scale motions (called turbulence). However, a growing body of experimental evidence indicates that there is often not a spectral gap in the atmospheric boundary layer, thereby raising questions about the Reynolds averaging approach that has formed the basis for turbulence theory for the past century.