What is the Specification pattern and how is it used in domain modeling?

Specification pattern encapsulates business rules as composable, reusable objects. Each specification has isSatisfiedBy(candidate) method. Specifications can be combined: AND (both must pass), OR (either passes), NOT (negation). Use cases: validation, querying (translate to SQL/NoSQL), eligibility checks. Benefits: reusable rules, testable in isolation, combinable for complex logic. Example: EligibleForDiscount = PremiumMember AND HasPurchasedRecently. Often combined with Repository for queries. Implementation: composite pattern for combinations.