Any stimulus with a discriminative function; according to an older usage, a stimulus correlated with reinforcement when another is correlated with extinction. The latter usage has become less common because it cannot easily be applied to stimuli correlated with different reinforcement schedules (e.g., multiple Fl FR); it was the source of SD ("S-dee". discriminative stimulus) and S6 ("S-delta". absence of discriminative stimulus) as abbreviations for the stimuli in a reinforcement-extinction discrimination. These abbreviations have lost ground to S' (positive stimulus) and S - (negative stimulus). Strictly, S0 ('S-zero") is more appropriate for the absence of reinforcement but S - is typographically more convenient.