Core loop is the innermost and most frequently repeated gameplay cycle: the sequence of actions (perceive → decide → act → receive feedback) that constitutes the game’s primary activity. In Clash of Clans, the core loop is: collect resources → spend on buildings/troops → attack → repeat. Every other loop (progression, social, meta) wraps around this inner cycle (Sellers 2018, see source-advanced-game-design).
The core loop must be intrinsically engaging — players will repeat it hundreds or thousands of times, so even small friction or reward-signal problems compound heavily.
Related: game-loops, interaction-loops, internal-economy, reward-systems, mda-framework