What is ACID (as in ACID transaction)

Atomicity, Consistency, Isolation and Durability

"a set of properties that guarantee that database transactions are processed reliably" ... wikipedia

Atomicity requires that each transaction be "all or nothing": if one part of the transaction fails, the entire transaction fails...

The consistency property ensures that any transaction will bring the database from one valid state to another...

The isolation property ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially....

Durability means that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors