ACID

Atomicity, Consistency, Isolation, Durability

Transaction guarantees in classic relational databases.

When you'd see it: Database decisions, especially when choosing between a traditional relational database and a NoSQL or distributed one.

Why it matters: ACID is the set of guarantees that makes a transaction trustworthy, so money does not vanish midway through a transfer. Systems that relax ACID trade that safety for scale or speed.

Common mistakes: Assuming every database is ACID-compliant. Many distributed and NoSQL systems are "eventually consistent," which is a different and weaker promise.