Play is essentially an activity which is enjoyed alone, though it can involve others, who perceive the play from their perspective and may not be in the mood for play. Play is most commonly associated with juvenile activities, and when engaged in by an adult they may be described as "childish" or "child at heart." Play can consist of an amusing, pretend or imaginary activity alone or with another.
Playing Children, by ChineseSong Dynasty artist Su Hanchen, c. 1150 AD.

A concerted endeavor has been made to identify the qualities of play, but this task is not without its ambiguities. For example, play is commonly perceived as a frivolous and non-serious activity; yet juveniles at play often display a transfixed seriousness and entrancing absorption while engaged in it. Other criteria of play include a relaxed pace and freedom versus compulsion. Yet play seems to have its intrinsic constraints, as in, "You're not playing fair."

When play is structured and goal-orientated it is often presented as a game. Play can also be seen as the activity of rehearsing life events, e.g., young animals play fighting. Play may also serve as a pretext, allowing people to explore reactions of others by engaging in playful interaction. Flirting is an example of such behavior. These and other concepts or rhetorics of play are discussed at length by Brian Sutton-Smith in the book The Ambiguity of Play. Sometimes play is dangerous, such as in extreme sports. This type of play could be considered stunt play, whether engaging in play fighting, sky-diving, or riding a device at a high speed in an unusual manner.

The seminal text in the field of play studies is Homo Ludens by Johan Huizinga. Huizinga defined 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, and attributed to Huizinga, who does make reference to the term at some points in Homo Ludens. According to Huizinga, within play spaces, human behavior is structured by very different rules: e.g., kicking (and only kicking) a ball in one direction or another, using physical force to impede another player (in a way which might be illegal outside the context of the game).

Another classic in play theory is Man, Play and Games by Roger Caillois. Borrowing much of his definition from Huizinga, Caillois coined several formal sub-categories of play, such as alea (games of chance) and ilinx (vertigo or thrill-seeking play).

According to Stephen Nachmanovitch, play is the root and foundation of creativity in the arts and sciences also as in daily life.

Improvisation, composition, writing, painting, theater, invention, all creative acts are forms of play, the starting place of creativity in the human growth cycle, and one of the great primal life functions.[7]

A notable contemporary play theorist is Jesper Juul who works on both pure play theory and the application of this theory to computer game studies. The theory of play and its relationship with rules and game design is also extensively discussed by Katie Salen and Eric Zimmerman in their book Rules of Play: Game Design Fundamentals. In computer games, the word gameplay is often used to describe the concept of play.

Symbolic play uses one thing to stand for another and shows the child's ability to create mental images. There are three types of symbolic play: dramatic play, constructive play, and playing games with rules.

Researchers at the National Institute for Play are creating a clinical, scientific framework for play. They describe seven patterns of play which indicate the range of activities and states of being which play encompasses.[6] References for each type of play are also listed.[6]

James Findlay, a Social Educator, defines play as "intelligence," suggesting further that play is the "meta intelligence" behind, together with, and changing, the various forms of intelligences people have. He argues that play intelligence is not a form of intelligence, it is intelligence in all its forms. He is known for his 3-dimensional models which demonstrate the principles of how intelligence and play function together.[8]