A stimulus following a response that increases the likelihood that the response will be repeated. go to glossary index

A reinforcer is anything following a behaviour that increases the chance of that behaviour happening again.

Anything that increases the strength of a behavior.

a pleasant contingency, or a happenstance, that occurs at the time a subject is doing something, or just when he/she has done it

In the context of ABA and discrete trial this means a reward or treat for getting the exercise right. It can be be anything as long as it's something the child really likes and wants. It could be a small morsel of a favourite food, or a tickle, or a squeeze. In mousetrial it's the funny animation that's the reinforcer.