Play in our species serves many valuable purposes. It is a means by which children develop their physical, intellectual, emotional, social, and moral capacities. It is a means of creating and preserving friendships. It also provides a state of mind that, in adults as well as children, is uniquely suited for high-level reasoning, insightful problem solving, and all sorts of creative endeavors. This essay is the first in a series I plan to post on The Value of Play. The subject of this first installment is the definition of play. Clues to play’s value lie in the definition.