I'm going to make this one a quick entry, what with the holidays and all, and hope that it expands into something worthwhile due to everyone's participation in the comments section.
I've nearly finished reading Rule of Play, by Katie Salen and Eric Zimmerman, and must add that this may be the best book on pure game design that I've yet read. In effect, this is the first book that I've seen that could be used as a college textbook on game design. It's well written, researched, and covers a great many topics from a fundamental angle that I've not seen done as well in other books.
One section of this book covers the definition of "game." The authors present their definition along with definitions from several other industry notables or academics. For those who haven't the book in front of them, here's some of the definitions of game from the book:
o "A game is a context with rules among adversaries trying to win objectives." -- Clark Abt
o "Playing a game is the voluntary effort to overcome unnecessary obstacles." -- Bernard Suits
o From Chris Crawford's The Art of Computer Game Design, he gives a lengthy definition that includes qualities of representation, interaction, conflict and safety. In part, he writes, "A game is a closed formal system that subjectively represents a subset of reality."
o "A game is a form of art in which participants, termed players, make decisions in order to manage resources through game tokens in pursuit of a goal." - Greg Costkyan
o "Games are an exercise of voluntary control systems, in which there is a contest between powers, confined by rules in order to produce a disequilibrial outcome." -- Elliot Avedon and Brian Sutton-Smith (from their book, The Study of Games)
o "A game is a system in which players engage in artificial conflict, defined by rules, that result in a quantifiable outcome." -- Katie Salen and Eric Zimmerman, from their book.
(Note: I do not include Sid Meier's informal definition, "a series of interesting decisions," because I do not think he meant this to be anything more than an off-the-cuff shorthand comment, even though I've seen him quoted dozens of times as if this was his gospel definition.)
All of these definitions generally work, but I'm a fan of simplicity. So, while reading these definition several months ago, I spent about five minutes to see if I could come up with a very short definition that still covers the key points. Here's what I ended up with: A game is a structured set of fun problems.
Here's why I like my definition: Conflict is not required, which means it includes games like The Sims, Bejeweled, Tetris, and flight simulators. Also, there's no requirement of a final outcome -- I do not require a game to be winnable. Costkyan talks about managing resources, but I disagree with this requirement because games like Frogger and Tetris don't have resources to manage -- they have a game piece to manipulate, but no resource to manage, and calling a game piece a resource is a real stretch. I include "structure" because a game must have rules and context. Lastly, I include the requirement of fun, which, strangely, most definitions I've seen leave out.
The heart of my definition is "fun problems". This is truly the essence of any game, and separates a game from any activity we don't prefer to do in our spare time. Problems can take the form of enemies, puzzles, hazards, conflict and anything that's a challenge. (If a game is not a challenge, then it's not fun, so challenge is built into the definition of fun.) Game designers do nothing else but throw problems at players, in one form or another, and successful games make problems fun to solve.
So, does my definition blow chunks? Got a better one? If so, let's hear it in the comments section...