ACID SQL stand for Atomicity, Consistency, Isolation and Durability.
Atomicity defines the way how the database handles changes in the database. If a part of the transaction fails, the entire transaction fails and the database remains untouched.
Consistency ensures the consistent state of the data throughout transactions. All write operations within the database need to be accepted with all predefined rules. It’s important to mention this is a database operation and do not guarantee consistency caused by faulty application level database operations.
Isolation ensures multiple transactions can be executed parallel as they were executed sequential. The content is not altered in the database, therefore a parallel query can be executed without interfering with the other query.
Durability ensures once the transaction has been completed, it’s committed into the database and will remain like this even when the servers encounters an outage.
Most relational databases (like Oracle and MSSQL) can be defined as ACID compliant.