Which term refers to when a stimulus increases the likelihood of a behavior occurring?

The term that refers to when a stimulus increases the likelihood of a behavior occurring is "reinforcer." In behavioral learning theories, particularly in operant conditioning, a reinforcer is any event or object that strengthens or increases the probability of a behavior that it follows. Reinforcers can be positive, where they involve the addition of a pleasant stimulus after a desired behavior, or negative, where a negative stimulus is removed following the behavior, thereby also increasing the likelihood of that behavior occurring again in the future.

The other terms mentioned have different functions within the framework of behavior analysis. An aversion typically refers to an unpleasant or negative stimulus that can lead to a decrease in behavior, rather than an increase. A discriminative stimulus signals the availability of reinforcement for a specific behavior, indicating when that behavior is likely to be reinforced, but it does not directly increase the likelihood of the behavior itself. Similarly, a conditioned stimulus is associated with a conditioned response in classical conditioning and does not necessarily increase the likelihood of a specific behavior occurring in the context of reinforcement. Thus, the concept of a reinforcer is central to understanding behavior modification and the principles of operant conditioning.