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A Pi-system is a concept from measure theory, a branch of mathematics that deals with the formalization of concepts like size and probability. A Pi-system (or π-system) is specifically a collection of sets that has some special properties: 1. **Closure Under Intersection**: If you have two sets \( A \) and \( B \) in the Pi-system, then their intersection \( A \cap B \) is also in the Pi-system.