An “ACID” database is a type of database that adheres to the principles of ACID, which is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. These principles are a set of properties that guarantee the reliability and integrity of database transactions. Here’s what each of these principles means:

1. Atomicity: Atomicity ensures that a transaction is treated as a single, indivisible unit of work. In other words, all the operations within a transaction are either completed successfully or none of them are. If any part of the transaction fails, the entire transaction is rolled back to its previous state, ensuring that the database remains in a consistent state.
2. Consistency: Consistency ensures that a transaction brings the database from one consistent state to another. It enforces certain integrity constraints, such as primary key uniqueness and foreign key relationships, to maintain the database’s integrity. If a transaction violates any of these constraints, it is rolled back.
3. Isolation: Isolation ensures that multiple transactions can be executed concurrently without interfering with each other. It guarantees that the result of one transaction is not visible to other transactions until the first transaction is complete. This prevents issues like “dirty reads,” “non-repeatable reads,” and “phantom reads.”
4. Durability: Durability ensures that once a transaction is committed, its effects are permanent and will survive any subsequent system failures, including power outages or crashes. Data changes made by committed transactions are stored in a way that they can be recovered and are not lost.

ACID properties are essential for databases that require high levels of data integrity, reliability, and consistency. Transactions in ACID-compliant databases are designed to protect data from corruption, provide predictable and reliable results, and maintain the database’s integrity.

Some relational database management systems (RDBMS) like PostgreSQL, Oracle, and SQL Server adhere to the ACID properties, but not all databases, especially NoSQL databases, follow these principles. The choice of whether to use an ACID-compliant database or a database with different consistency and reliability characteristics depends on the specific requirements of an application.