Puzzles, curiosities, brain teasers and other mathematics done "just for fun".
Recreational mathematics is a general term for mathematical problems studied for the sake of pure intellectual curiosity, without necessarily having any practical application or expectation of deep theoretical results.
Recreational mathematics problems are often easy to understand even for people without an extensive mathematical education, even if the theory they lead to may turn out to be surprisingly deep. Thus, recreational mathematics can serve to attract the curiosity of non-mathematicians and to inspire them to develop their mathematical skills further.
Many typical recreational mathematics problems fall into the fields of discrete mathematics (combinatorics, elementary number theory, etc.), probability theory and geometry.