Unconditioned Reinforcer refers to a stimulus that naturally increases the likelihood of a behavior occurring in the future without the need for prior learning or pairing with other reinforcers. These reinforcers are inherently satisfying and do not require conditioning to be effective.
Example
Food is an unconditioned reinforcer for a person who is hungry. Eating food naturally increases the likelihood of behaviors that lead to obtaining food in the future, without needing to learn that food is rewarding.