Something which increases the chance that the behavior immediately preceding it will be repeated. Example: My dog looks at me and I give her a piece of food. If her behavior ("looking at me") increases, then the piece of food is a positive reinforcer. Warning--your dog tells you what can act as a primary reinforcer; you can't decide. For some dogs, it might be a cheerio; others like popcorn; others like pieces of hot dog or cheddar cheese.