What is a CASB (Cloud Access Security Broker)?

Cloud access security brokers (CASB) are on-premises or cloud-hosted software that sit between cloud service consumers and cloud service providers to enforce security, compliance, and governance policies for cloud applications. CASBs help organizations extend the security controls of their on-premises infrastructure to the cloud.

Reference: https://www.skyhighnetworks.com/cloud-security-university/what-is-cloud-access-security-broker/