Final answer:
A stimulus that strengthens a response when presented after that response is known as a positive reinforcer in the context of operant conditioning.
Step-by-step explanation:
Any stimulus that, when presented after a response, strengthens the response is called a positive reinforcer. This concept is an integral part of operant conditioning, a method of learning that employs rewards and punishments to influence behavior. A positive reinforcer implies the addition of a desirable or pleasant stimulus following a behavior, increasing the likelihood that the behavior will occur again in the future. This is different from a negative reinforcer, which involves the removal of an aversive stimulus to increase the desired behavior.