What is a stimulus that encourages the likelihood of a response called?

A stimulus that encourages the likelihood of a response is called a reinforcer. In psychology, particularly in the context of operant conditioning, a reinforcer is any stimulus that increases the probability of a behavior being repeated. When a behavior is followed by a positive outcome or reward, it reinforces that behavior, making it more likely to occur again in the future.

For instance, if a student studies hard and receives praise from a teacher, the praise serves as a reinforcer. The positive association encourages the student to continue studying in order to receive more praise.

The other types of stimuli mentioned have different roles: a punisher decreases the likelihood of a behavior, a neutral stimulus does not elicit a response by itself, and a conditioned stimulus is one that has been associated with an unconditioned stimulus to elicit a response. Therefore, the term reinforcer specifically relates to any stimulus that increases the probability of a desirable response.