ACID stands for Atomicity, Consistency, Isolation, Durability. These properties ensure that all operations within a transaction are treated as a single unit (Atomicity), transactions preserve the integrity of the database (Consistency), concurrent transactions do not interfere with each other (Isolation), and once a transaction is committed, its changes are permanent (Durability). Delta Lake leverages these ACID properties to ensure data reliability, consistency, and durability in a data lake environment.