I've been searching for articles online and came across a book written in 1938 by a Dutch historian and cultural theorist, Johan Huizinga. In his book "Homo Ludens" (Man the Player) he defines five characteristics of play. Though this text is dated, I thought his points were interesting since I don't think we've come up with our own definition of play.
1. Play is free, is in fact freedom
2. Play is not "ordinary" or "real" life
3. Play is distinct from "ordinary" life both as to locality and duration
4. Play creates order, is order. Play demands order absolute and supreme.
5. Play is connected with no material interest, and no profit can be gained from it
Something to think about...
P.S. see the wikipedia entry for more information: http://en.wikipedia.org/wiki/Homo_Ludens