The ACID properties are a set of principles that ensure reliable processing of database transactions. They stand for Atomicity, Consistency, Isolation, and Durability. Atomicity guarantees that a transaction is all-or-nothing, meaning it either completes fully or not at all. Consistency ensures that a transaction brings the database from one valid state to another, maintaining all predefined rules. Isolation ensures that transactions occur independently without interference, even if they are executed simultaneously. Finally, Durability guarantees that once a transaction is committed, it remains permanent, even in the event of a system failure. Together, these properties help maintain data integrity in databases.