A film whose thickness is of the order of a characteristic scale or smaller. Since a film may 'look' operationally thin or thick, according to the procedure applied, it is also recommended that the measurement procedure employed be specified (e.g. ellipsometrically thin film, optically thin film, etc.). It is recommended that the physical specification of the film thickness be used, whenever possible (e.g. thick compared to the electron mean free path, thin compared to the optical wavelength, etc.)