having an internal cause or origin; growing or originating from within an organism; (of a disease or symptom) not attributable to any external or environmental factor. (first page Google results)
Greg Costikyan says games have endogenous structure, meaning it's important only within the context of the game. Monopoly money is very important while playing Monopoly and worthless outside the game. This idea and term are an excellent measure of how compelling a game really is. For example, if it can be played with just tokens instead of real money at stake, the game is compelling. Some MMOs have such great endogenous value that people trade items for real money.
Game Design: The Lens of Endogenous Value
To use this lens, think about your players' feelings about items, objects, and scoring in your game.
- What is valuable to the players in my game?
- How can I make it more valuable to them?
- What is the relationship between value in the game and the player's motivations?
The value of the items and score in the game is a direct reflection of how much players care about succeeding in your game. By thinking about what the players really care about and why, you can often get insights about how your game can improve.
(The Art of Game Design: A Book of Lenses by Jesse Schell, Carnegie Mellon University, 2008. Page 32)
Example
Bubsy (SNES & Sega Genesis) is a fairly standard platform game. Points serve no purpose other than to measure how many things you have collected - no in-game reward. Most players collect yarn balls at first, with the expectation that they are valuable, but after playing a short while, they completely ignore them, focusing only on defeating enemies, avoiding obstacles, and fetting to the end of the level. Why? The player's motivation (The Lens of Curiosity) is merely to complete the levels. A higher score doesn't help that.
Sonic the Hedgehog 2 (Sega Genesis) was a similar platform game but did not have this problem. The number of rings Sonic collects has a lot of endogenous value. They protect you from enemies, and every time he collects one hundred rings, he receives an extra life.
(The Art of Game Design: A Book of Lenses by Jesse Schell, Carnegie Mellon University, 2008. Page 33)