What is ACID in the context of database transactions, decode each letter and what does it provide to the application?

ACID is a set of properties that guarantee reliable transactions in a database:

• Atomicity — a transaction is completed in full or not at all. If something goes wrong, all changes are rolled back.
• Consistency — after the transaction completes, the database remains in a correct state, with all rules and constraints satisfied.
• Isolation — parallel transactions do not affect each other, resulting in the same outcome as if they were executed sequentially.
• Durability — after a transaction is committed, changes are permanently saved, even in case of system failures.

For an application, this means that data will always be consistent and reliable, even in case of failures or simultaneous work by many users. For example, when transferring money between accounts, either both accounts are updated, or none — excluding partial changes.