Atomicity refers to the concept that transactions need to be atomic. Either it succeeds and its results are visible to every subsequent user reading them, or it fails and every change is rolled back to the point it was at before it started. Either all actions in a transaction occur, or none at all.

A simple example to understand atomicity is by transferring money from account A to account B. Money needs to be credited from account A and then debited into account B. If the operation fails midway, then both accounts A and B need to be reverted to their state before the operation started.

In MongoDB, operations in a single document are always atomic even if the operation spans multiple subdocuments or arrays within the document.

Operations spanning multiple documents need to use MongoDB transactions to be made atomic.