ACID

ACID names the four guarantees a transactional database provides: atomicity — the whole transaction happens or none of it does; consistency — the rules of the schema always hold; isolation — concurrent transactions do not see each other half-finished; durability — once confirmed, it survives a crash.

Read them as a list of the disasters they prevent. Without atomicity, money leaves one account and does not arrive in the other. Without isolation, two simultaneous bookings take the same seat. Without durability, a confirmed order disappears when the power fails.

These are not free, and some distributed systems relax them to gain speed or scale. That is a legitimate engineering choice, but it must be a deliberate one — the guarantees you drop become code somebody has to write, and bugs somebody has to find.