Play is a term employed in psychology and ethology to describe voluntary activities associated with recreational pleasure and enjoyment. Play is usually connected with children and their juvenile-level activities, but it can also be a useful adult activity, and occurs among other higher-functioning animals as well.
Many of the most prominent researchers in the field of psychology (including William James, Sigmund Freud, Carl Jung, Jean Piaget and Lev Vygotsky) have viewed play as endemic to the human species. Play may be frivolous; but it may be serious. A player can be intently focused on his or her objective (in the zone), particularly when play is structured and goal-oriented, as in a game. Accordingly, play can range from relaxed, free-spirited and spontaneous through frivolous to planned or even compulsive.
There have been two major reviews of research into animal play, and both have said it is extremely difficult to define. Play is almost universal in very young mammals, and also in some groups of birds such as parrots and ravens. Therefore, it must have a function, but what is that function? It is definitely not immediate survival. In fact, the young animals of many species may risk their lives playing.preface Yet, since it survives in so many species, it must have survival value. That, as Fagan says, is a paradox.
- "The problem of defining play is one of the greatest challenges facing neurology, behavioral biology, psychology, education and the social sciences generally". Burghardt.preface
The main idea is that play is basically useful, and so it can be explained by the normal process of evolution by natural selection. This was proposed by Karl Groos over a century ago, and it is still the majority explanation.
Children's play [change]
Play can take the form of improvisation or pretend, performance, mimicry, games, sports. Play can be structured, with goals and rules; when this is the case, the play is a "game". Other play is unstructured, without rules. The gurus of children's games were Iona and Peter Opie, who collected them in England in the post-World War II period.
Often sports with specific rules will take place within designated play spaces, such as sports fields. Other play spaces may be playgrounds with equipment and structures to promote active and social play. Some play spaces may be indoors, as with museums, or family entertainment centres.
The National Institute for Play describes seven play types: 
- Attunement, which establishes a connection, such as between newborn and mother.
- Body, in which an infant explores the ways in which his or her body works and interacts with the world, such as making funny sounds or discovering what happens in a fall.
- Object, such as playing with toys, banging pots and pans, handling physical things in ways that use curiosity.
- Social, play which involves others in activities such as tumbling, making faces, and building connections with another child or group of children.
- Imaginative (also called "pretend" or "fantasy"), in which a child invents scenarios from his or her imagination and acts within them as a form of play, such as princess or pirate play.
- Narrative (or storytelling), the play of learning and language that develops intellect, such as a parent reading aloud to a child, or a child retelling the story in his or her own words.
- Transformative. This means play with imagination to create a higher mental state. For example, a person might experiment to find a new way to use a musical instrument; or, as Einstein was known to do, a person might wonder about things which are not yet known and play with unproven ideas as a bridge to discover new knowledge.
- Garvey C. 1990. Play. Cambridge, MA: Harvard University Press.
- Huizinga J. 1955. Homo Ludens; a study of the play-element in culture. Boston, Beacon Press
- Fagan, Robert 1981. Animal play behavior. Oxford University Press. ISBN 0-19-502761-2
- Burghardt, Gordon M. 200. The genesis of animal play: testing the limits. MIT Press. ISBN 978-0-262-52469-8
- Groos, Karl 1898. The play of animals (transl. E.L. Baldwin). New York: Appleton.
- Opie, Iona and Peter 1959. The lore and language of schoolchildren. Oxford University Press.
- Opie, Iona and Peter 1969. Children's games in street and playground. Oxford University Press
- Avery, Gillian and Julia Briggs (eds) 1989. Children and their books: a celebration of the work of Iona and Peter Opie. Oxford University Press.
- National Institute for Play. "Play Science – the patterns of play". http://www.nifplay.org/states_play.html. Retrieved 2012-01-16.
- Piaget J. 1962. Play, dreams and imitation (Vol. 24). New York: Norton
- Auerbach, Stevanne 2004. Dr. Toy's smart play smart toys: how to raise a child with a high PQ (play quotient). ISBN 1-56767-652-9