What is the DRY principle?

DRY (Don't Repeat Yourself) states that every piece of knowledge should have a single, unambiguous representation in the system. Avoid duplicating code, logic, or data - extract common functionality into functions, classes, or modules. Benefits: easier maintenance (change in one place), fewer bugs (no inconsistent copies), smaller codebase. Counter: YAGNI (don't over-engineer abstractions). WET (Write Everything Twice) is anti-pattern. Balance DRY with readability and simplicity.