1) Reinforcers per unit time; sometimes used in preference to reinforcement frequency because frequency occasionally refers not to reinforcers per unit time but rather to reinforcers per response, per session, or per trial.

2) Responses per unit time, usually responses per second. It is convenient to distinguish between different rates according to the interval of time covered.

3) Total reinforcers, over a fixed time, over a session of variable duration, over a fixed number of responses or, in a trial procedure, over a fixed number of trials. With respect to reinforcement, frequency is most commonly a synonym of rate; the usage for responding is more likely to be variable.