Guaranteed properties of many database systems - This is an acronym for Atomicity, Consistency, Isolation, Durability.
ACID (atomicity, consistency, isolation, durability) is a set of properties that guarantees that database transactions are processed reliably. This means that a database server that is ACID compliant can guarantee that it has not violated any constraints, even when a transaction fails to complete.
Most SQL-based systems are intended to be ACID-compliant. NoSQL systems, however, typically do not guarantee this. (They are usually attempting for This database terminology makes assurances about the data state which are BASE (Basically Available, Soft state, Eventual consistency).
Atomicity : (All at Once) A transaction is completely executed, or if a part of the transaction fails, no changes are made.
Consistency : (Never Breaks Rules) At no point is the database violating any of the constraints that exist in the system. This means that the system cannot implement constraints to occur after the data is written - it must guarantee that the current database state is always allowed.
Isolation : (One at a time) If two database operations occur, they must either have a defined order (One transaction occurs before the other), or the order of the operations must be irrelevant to the transactions.
Durability : (When it's done, it's done.) Once a transaction is complete, any modified, new, or removed data is now in the database, and it stays unless further modified. The transaction cannot be undone by, for instance, a power failure.
