The terms substrate and subphase refer to a condensed phase that physically supports a film or layer. The term substrate should be used with care in the light of the existing double meaning of the term substrate in film science and technology, on the one hand, and in biochemistry, on the other. The term subphase applies, mainly, to a liquid phase underlying an adsorption layer or a floating spread monolayer.
Source:
PAC, 1994, 66, 1667. 'Thin films including layers: terminology in relation to their preparation and characterization (IUPAC Recommendations 1994)' on page 1671 (https://doi.org/10.1351/pac199466081667)