Wiktionary
recreational mathematics
n. (context mathematics English) Any use of mathematics or logic whose primary purpose is recreation, though often with more serious characteristics.
Wikipedia
Recreational mathematics
Recreational mathematics is a term for mathematics carried out for recreation (entertainment) rather than as a strictly research and application-based professional activity. Although, it is not necessarily limited to being an endeavor for amateurs. It often involves mathematical puzzles and games.
Many topics in this field require no knowledge of advanced mathematics, and recreational mathematics often appeals to children and untrained adults, inspiring their further study of the subject.