Play is a range of intrinsically motivated activities done for recreation. Play is commonly associated with children and juvenile-level activities, but may be engaged in at any life stage, and among other higher-functioning animals as well, most notably mammals and birds.

Play is often interpreted as frivolous; yet the player can be intently focused on their objective, particularly when play is structured and goal-oriented, as in a game. Accordingly, play can range from relaxed, free-spirited, spontaneous, and frivolous to planned or even compulsive. Play is not just a pastime activity; it has the potential to serve as an important tool in numerous aspects of daily life for adolescents, adults, and cognitively advanced non-human species (such as primates). Not only does play promote and aid in physical development (such as hand-eye coordination), but it also aids in cognitive development and social skills, and can even act as a stepping stone into the world of integration, which can be a very stressful process. Play is something that most children partake in, but the way play is executed is different between cultures, and the way that children engage with play varies.

The seminal text in the field of play studies is the book Homo Ludens first published in 1944 with several subsequent editions, in which Johan Huizinga defines play as follows:

Summing up the formal characteristic of play, we might call it a free activity standing quite consciously outside "ordinary" life as being "not serious" but at the same time absorbing the player intensely and utterly. It is an activity connected with no material interest, and no profit can be gained by it. It proceeds within its own proper boundaries of time and space according to fixed rules and in an orderly manner. It promotes the formation of social groupings that tend to surround themselves with secrecy and to stress the difference from the common world by disguise or other means.

This definition of play as constituting a separate and independent sphere of human activity is sometimes referred to as the "magic circle" notion of play, a phrase also attributed to Huizinga. Many other definitions exist. Jean Piaget stated, "the many theories of play expounded in the past are clear proof that the phenomenon is difficult to understand."

Another definition of play from the twenty-first century comes from the National Playing Fields Association. The definition reads as follows: "play is freely chosen, personally directed, intrinsically motivated behaviour that actively engages the child." This definition focuses more on the child's freedom of choice and personal motivation related to an activity.

Play can take the form of improvisation, pretense, interaction, performance, mimicry, games, sports, and thrill-seeking (including extreme or dangerous sports like sky-diving, high-speed racing, etc.). Philosopher Roger Caillois wrote about play in his 1961 book Man, Play and Games. Caillois interprets many social structures as elaborate forms of games and much behaviour as a form of play.

Free-form play gives children the freedom to decide what they want to play and how it will be played. Both the activity and the rules are subject to change in this form, and children can make any changes to the rules or objectives of the play at any time. Some countries in the twenty-first century have added emphasis of free play into their values for children in early childhood, for example Taiwan and Hungary.