MDA framework
Also said as Mechanics, dynamics, aesthetics, MDA
A way of seeing a game in three layers: mechanics are the rules, dynamics are what those rules do when played, and aesthetics are how it feels. It comes from a 2004 paper by Robin Hunicke, Marc LeBlanc and Robert Zubek.
Designers can only touch the rules and players only feel the top layer, with dynamics between them. It explains why a small rule change can transform a game, and it replaces the word "fun" with eight more exact kinds: sensation, fantasy, narrative, challenge, fellowship, discovery, expression and submission.
Say it
“The feeling we want is a tense race right to the end. Dynamically that means the lead has to keep changing. So the mechanic to try is taxing the leader on each lap, about 20%.”
Where you’ve seen it
The paper's own example is Monopoly: rents make the rich richer, the gap widens, and soon only one or two players are really invested. Its suggested fixes are at the level of mechanics: subsidies for poor players, and taxes for rich ones.
Not to be confused with aesthetics as art style
In MDA, aesthetics means the emotional response, such as challenge or fellowship. It has nothing to do with how the game looks.