(in chromatography) the power to resolve individual compounds (see resolution(def. 1)). It depends on (1) the efficiency of the system employed; for columns this is given by the theoretical platenumber; and (2) the relative affinities of different compounds for the stationary and mobile phases.
(in physics) (a) the ability of an optical instrument, e.g. a microscope or telescope, to produce separate images of objects placed close together.
(b) the ability of a spectrometer to separate adjacent peaks in a spectrum.