What is Gameplay?

My personal theory of fun centers around gameplay. I don't think that gameplay is the only reason why we play games, and I certainly don't think it's the only reason why we enjoy interactive or digital media. However, it is the clear point of distinction between games and other forms of media (books, movies, interactive movies), and it's an aspect of games that is the primary responsibility of game designers. And because that's my job, I selfishly consider it a  fundamental aspect of fun.

So, what is gameplay?

First, I think it's important to define clearly what a game is, and to differentiate it from other game-like things. To me, the critical aspects of a game are:

• Games are played by one or more players. If something doesn't have a player, it's not a game (for example, an a-life demo isn't a game, even though it has states and rules and an idea of failure).
• Games are, fundamentally, big state machines. This means that you have some idea of game state that is updated by actions taken by players. The game state can be represented by anything from a board and pieces, to a 3D rendered world on a monitor, to players on a field, to some nerds running around with foam swords hitting each other.
• Games always include some sort of metric for player performance. This means that games are, essentially, goal oriented. While some games may not have winners or losers (e.g. D&D) they do present metrics or mechanics that reward players for successful play (XP in the case of D&D) or punish them for unsuccessful play (Death in the case of D&D).
• Games have rules, be they simulative (e.g. Forza Motorsport) or more abstract (Chess, Tetris).
• There are two types of rules: first, a series of actions are defined as valid for players. These can be presented as the valid inputs allowed to players in a video game, or as a list of things you can't do in a sport, or a list of how each piece moves in a board game.
  
  Second, you have rules that determine how the game state is updated based on the cumulative actions of players. These rules can be relatively simple (Chess) or very complex (American Football).

Given that definition of a game, I would then define gameplay as the feedback loop between the actions of one or more players and the rules, with each player (or the only player, in the case of a single-player game) attempting to make choices that optimize their outcome in whatever success metric the game uses. That feedback loop relies on players understanding the actions available and forecasting the effects of actions to determine which will be most or least successful.

I think this definition is helpful because it starts to take apart things that have traditionally been a part of games (e.g. Puzzles) and sets them apart from core gameplay. Most puzzles in games don't really rely on gameplay - when you're trying to find the right item to open a door in an adventure game, there isn't really a ruleset to rely on or a gamestate to update. If there is, it's so simplistic (Actions: each item in your inventory! Rules: binary success or failure based on which item you picked!) that the "gameplay" becomes trivial. Or, the results of the actions are unclear, and the goal is to figure out which action is correct, not to optimize the results across many different valid options.