Deceptive agents are a challenge for the safety, trustworthiness, and cooperation ofAI systems. We focus on the problem that agents might deceive in order to achievetheir goals. There are a number of existing definitions of deception in the literatureon game theory and symbolic AI, but there is no overarching theory of deceptionfor learning agents in games. We introduce a functional definition of deceptionin structural causal games, grounded in the philosophical literature. We presentseveral examples to establish that our formal definition captures philosophical andcommonsense desiderata for deception.