The ACID acronym stands for:

• Atomicity: All operations in a transaction either succeed or all are rolled back.
• Consistent: The database integrity constraints are valid on completion of the transaction.
• Isolated: Simultaneously occurring transactions do not interfere with each other. Contentious concurrent access is moderated by the database so that transactions appear to run sequentially.
• Durable: Irrespective of hardware or software failures, the updates made by the transaction are permanent.

This is the most stringent of all consistency models. Most developers are familiar with and rely on the ACID properties of databases. Coding is so much easier when a data store offers this consistency, but the tradeoff is that such systems usually don't scale well (covered in Chapter 8, Modeling Data).