Emergence occurs when the interaction of a system’s parts produces behaviour that cannot be predicted from the parts in isolation. In games, emergent gameplay arises when players discover combinations, strategies, or situations the designer never explicitly scripted (Sellers 2018, see source-advanced-game-design). Chess has simple rules but an effectively infinite space of emergent play.
Emergence is both a goal (rich, replayable systems) and a risk (unintended exploits, dominant strategies). second-order-design is the practice of deliberately engineering for emergence rather than scripting paths.
Related: systems-thinking, systemic-depth-elegance, second-order-design, mda-framework, game-loops