1. n. a logical antecedent on which a conclusion is dependent or an empirical antecedent on which an event or state is dependent. A necessary condition is one without which the idea would not logically follow or the event would not occur. A sufficient condition is one that directly entails a particular conclusion or that has the power to produce a particular event regardless of other conditions.

2. n. in statistics and experimental research, a category or level of a variable whose values are manipulated by a researcher. Study participants are then assigned to receive or be exposed to one or more of the different conditions.

3. vb. to inculcate a response or a behavior in an organism by means of classical conditioning, operant conditioning, or other behaviorist paradigms (see behaviorism). The term implies that the learning is largely automatic, based on processes more like reflexes than conscious mental activity. —conditional adj.