These four principles are referred to as ‘ACID’, and each letter is an acronym for one property of RDBMS systems that is non-negotiable for the sake of the integrity of the system.
Atomic(ity) – The principle that each transaction is ‘all-or-nothing’, i.e. it either succeeds or it fails, regardless of external factors such as power loss or corruption. On failure or success, the database is left in either the state in which it was in prior to the transaction or a new valid state. The transaction becomes an indivisible unit.
Consistency – The principle that the database executes transactions in a consistent manner, obeying all rules (constraints).
Isolation – This property means that each transaction is executed in isolation from others, and that concurrent transactions do not affect the transaction.
Durability – This property means that the data written to the database is durable, i.e. it is guaranteed to be in storage and will not arbitrarily be lost, changed or overwritten unless specifically requested. More formally, it means that once a transaction is committed, no event can ‘un-commit’ the transaction – it is written and cannot be changed retrospectively unless by another transaction.