ACID stands for atomicity, consistency, isolation, and durability:

• Atomicity: This indicates that database transactions either execute in full or do not execute at all. In other words, either all transactions should be committed, that is, persisted in their entirety, or not committed at all. There is no scope for a partial execution of a transaction.
• Consistency: The constraints on the data, that is, the rules that determine data management within a database, will be consistent throughout the database. Different instances will not abide by rules that are any different to those in other instances of the database.
• Isolation: This property defines the rules of how concurrent operations (transactions) will read and write data. For example, if a certain record is being updated while another process reads the same record, the isolation level of the database system will determine which version of the data would be returned back to the user.
• Durability: The durability of a database system generally indicates that committed transactions will remain persistent even in the event of a system failure. This is generally managed by the use of transaction logs that databases can refer to during recovery.

The reader may observe that all the properties defined here relate primarily to database transactions. A transaction is a unit of operation that abides by the aforementioned rules and makes a change to the database. For example, a typical cash withdrawal from an ATM may have the following logical pathway:

1. User withdraws cash from an ATM
2. The bank checks the current balance of the user
3. The database system deducts the corresponding amount from the user's account
4. The database system updates the amount in the user's account to reflect the change

As such, most databases in popular use prior to the mid-1990s, such as Oracle, Sybase, DB2, and others, were optimized for recording and managing transactional data. Until this time, most databases were responsible for managing transactional data. The rapid growth of the internet in the mid-90s led to new types of data that did not necessarily require the strict ACID compliance requirements. Videos on YouTube, music on Pandora, and corporate email records are all examples of use cases where a a transactional database does not add value beyond simply functioning as a technology layer for storing data.