What is an ACID transaction?

ACID is an acronym for properties of a transaction like Atomicity, Consistency, Isolation and Durability.
Atomicity: Atomicity means all or nothing in transactions. It means that when a transaction is performed either all transaction actions are visible to another transaction or none are visible.
Consistency: This property is to do with maintaining the integrity of a transaction. A transaction is consistent if it leaves the database in a consistent state after it completes or rollback the changes and bring the database to the state that it was in before the transaction started.
Isolation: Isolation means that the transactions takes place in isolation i.e. if two transactions are executing none of them interfere with one another. i.e. two concurrent transactions do not see each others updates.
Durability: It means that the changes made by a transaction once it is complete will be visible in the database to the other transactions until hey are overwritten as a result of another transaction.