Definition
ACID is the acronym for the four properties a database transaction is expected to provide. Atomicity means the transaction's writes all take effect or none do. Consistency means it moves the database from one valid state to another, respecting declared constraints. Isolation means concurrent transactions do not observe each other's partial work. Durability means once committed, the changes survive a crash.
The word doing the most work is isolation, because full isolation (serializability) is expensive and most databases default to something weaker. Two systems can both advertise ACID and still behave differently under concurrency depending on the isolation level in use. The C is also the least interesting letter: it is a property of the application's constraints rather than something the engine provides on its own, and it is unrelated to the C in the CAP theorem.
How it comes up in interviews
Reciting the acronym earns nothing. What earns credit is knowing the default isolation level of the database you named and what anomaly it still permits, and being able to say which parts of your design genuinely need a transaction versus which are fine with idempotent retries and eventual convergence.