A discrete period, usually stimulus-correlated, during which an organism has an opportunity to respond. Trials are separated by intertrial intervals that may consist of any of the following. a stimulus condition (e.g., a dark chamber); removal of the operandum (or operanda); or removal of the organism from the chamber (especially when the organism, after en-dtting a response such as running an alley, is no longer in a position to respond again). Trials distinguish discrete-operant procedures from free-operant procedures.