A Cumulative Update (CU) is a Microsoft software package that consolidates multiple updates - such as security patches, bug fixes, and feature enhancements - into a single installation. Unlike individual updates, a CU contains all previous updates released since the last major version or service pack, ensuring that systems are fully up to date with the latest improvements and protections. CUs simplify the update process for administrators by reducing the number of separate patches that need to be installed and tested. They are commonly used in products like Windows, SQL Server, and Exchange Server, and are released on a regular schedule, helping organizations maintain security and stability while minimizing deployment complexity.