A @[email protected] whose thickness is of the order of a @[email protected] or smaller. Since a @[email protected] 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 @[email protected], optically thin @[email protected], etc.). It is recommended that the physical specification of the @[email protected] thickness be used, whenever possible (e.g. thick compared to the electron @[email protected], thin compared to the optical @[email protected], etc.)