Semantic encapsulation inside a domain defining where a term has one meaning. Not a natural boundary — it is a design decision. Small contexts fit in one head but require interaction design; a single huge context eliminates interaction but becomes unlearnable. Each bounded context owns its own model (e.g., customer in payment = creditor/debitor; in delivery = shipping address; warehouse may not know 'customer' exists). Contexts share only identifiers (e.g., a customer reference / UUID). Candidates surface via language shifts, domain-expert clustering, data cohesion, business process steps (drafting vs. ordered states), and bottlenecks. In source code typically expressed as a namespace, project, or gem. Bounded contexts are NOT microservices, though sometimes they coincide.