ACID stands for Atomicity, Consistency, Isolation, and Durability. These are a set of characteristics that are desirable for database transactions. Together, they guarantee that transactions and all data stored in the database remains valid even in the event of errors or power failures. These characteristics are extremely important for any OLTP database.
Atomicity: Each transaction must be treated as a single, indivisible unit (even if processing the transaction requires multiple steps). In other words, every step of the transaction must complete successfully or the entire transaction will fail and no change will be written to the database. This is desirable because without atomicity, a transaction that’s interrupted while processing may only write a portion of the changes it’s supposed to make, which could leave the database in an inconsistent state.
Consistency: No transaction can violate the integrity of the database – all transactions must leave the database in a valid state.
Isolation: Any concurrently-processed transactions (i.e. transactions happening at the same time) leave the database in the same state as if they were executed sequentially (i.e. one after another).
Durability: Once committed, the transaction remains committed even in the case of hardware failure, power outage, etc.