What do the ACID properties mean?

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ACID is the set of properties that make database transactions reliable. Atomicity means partial completion is impossible - the transaction either finishes fully or not at all. Consistency means every transaction moves the database from one valid state to another, respecting all constraints. Isolation means transactions running in parallel do not see each other's uncommitted changes. Durability means once you COMMIT, the data is safe even if the server crashes immediately after. Different isolation levels trade some isolation for higher concurrency - read committed, repeatable read, and serializable are the common options.