An important database term is the acronym ACID. But what does this term mean?

ACID stands for Atomicity, Consistency, Isolation, and Durability, and the elements consists of:

Atomicity
This means that transactions are atomic and indivisible. Either all of a transaction's changes are stored in the database, or none of them are stored. In the event of an external error, it is obviously ideal if the recovery process can complete any transactions that were in progress at the time; however, it is also acceptable for those transactions to be completely rolled back.

Consistency
This means that operations transform the database from one valid state to another. There are no intermediate stages where the data is inconsistent. The database should also disallow operations that violate consistency constraints. If you are storing bank accounts that relate to bank customers, it should not be possible to create an account for a customer who does not exist, and it should not be possible to delete a customer from the customers table if there are still accounts referring to them in the accounts table.

Isolation
This means that transactions do not affect each other while they are running. Each transaction should be able to view the world as though it is the only one reading and altering things. In practice this is not usually the case, but locks are used to achieve the illusion. Depending on the database and option settings, you will have different levels of isolation in practice. (See the "Transaction Isolation" section in this chapter for more detail.)

Durability
This means that after a transaction has been committed to the database, its effects are permanent. This would be a fairly simple requirement to satisfy in a simple program, but in a complex RDBMS that uses locking and multiversioning to allow concurrent multiuser access and caching to improve performance, it is a minefield. In addition, durability implies that we should be able to recover the current state of the database in the event of a failure. If a power failure, hard-disk crash, or other catastrophe occurs between a client sending a transaction to the database and that transaction being recorded on disk, then we should be able to combine a backup and a log to bring the database back to its precrash state and perhaps process transactions that had been logged but not yet executed or committed.