Atomicity: either all parts of the transaction complete, or all parts of the transaction don't complete; there's no "partly complete" state

Consistency: the state of the database after a transaction is stable; all "downstream" effects (e.g. triggers) of the query are complete before the transaction is confirmed.

Isolation: concurrent transactions behave the same as sequential transactions

Durability: a power failure or crash won't lose any transactions

Traditionally, ACID is where relational databases shine.

You've gotten good answers from other folks but I'll provide a ELI5:

Basically a set of rules in the database to make sure that it is immediately consistent.

NoSQL databases offer eventual consistency in exchange for speed so they are generally not considered to be ACID compliant.

Most traditional databases (MySQL, postgresql, etc.) are.

There are a couple of emerging companies that try to tackle speed for traditional databases. CockroachDB offers a postgress-based database that scales more like NoSQL while still offering ACID transactions.

TiDB is a similar company but for MySQL.

https://en.m.wikipedia.org/wiki/ACID

Atomicity (something happens in its entirety or not at all), consistency (database is always in a valid state

if the database has constraints, they will always be honored), isolation (transactions don't step on each other), durability (complete transaction is complete even if there's a power failure).

Not a database expert, my parenthetical explanations may need work.