Structural coupling (Maturana & Varela, via Sellers) describes the ongoing mutual adaptation between a player and a game system: as the player’s mental model changes through play, their behaviour changes; as their behaviour changes, different parts of the system are activated; the effective game the player experiences shifts accordingly. A beginner and an expert playing the same game are, in a meaningful sense, playing different games (Sellers 2018, see source-advanced-game-design).
Structural coupling is why game design cannot be evaluated in isolation from the player: the game and player co-produce the experience together over time.
Related: systems-thinking, fun-as-learning, flow, chunking, second-order-design