ACID stands for
Modification on the data in the database either fail or succeed. The beginning of such a modification starts with a transaction and ends when a transaction finishes (either by a commit or a rollback). A software crash entails an implicit rollback.
Only valid data (valid according to integrity constraints) may be committed.
One transaction does not interfere with another. The ‘executor’ of a transaction has the feeling that he has the entire database for himself.
A committed transaction will not be lost.