Try though I may I have always found a definition of play to be elusive. It is like trying to define love. It is different things to different people. It changes. It is full of layers. This piece of paper is hanging on my studio wall. I took it from the General Comment (#17) on article 31 of the UN Convention on the Rights of the Child. It is about as close to spot on as I have ever found. The addition to "designers" is mine. I consider my role as a designer of play spaces to be akin to that of caretaker, for what are designers doing but taking care to provide a place capable of supporting the magic of play?
I think I'll leave this on the wall until I have it memorized.