what is ACID

Atomicity—Atomicity is the property of an operation being atomic,or, in other words, all or nothing. If the operation succeeds, the entire operation succeeds. If the operation fails, it fails in its entirety and the system is left in exactly the same state as it was in before the operation started.
Consistency—Consistency is the property of an operation taking the system from one  valid  state  to  another.  If  the  operation  makes  the  system  inconsistent,  it won’t be performed or it will be rolled back.
Isolation—Isolation  means  that  no  operation  interferes  with  any  other  operation in the system. For instance, no two writes to a single object will happen at the same time. The writes will happen one after the other, but not at the exact same moment.
Durability—Durability is something we talked about earlier. It means that once data is written, it’s guaranteed to be read back and not lost in due course of normal operation of the system.