Identity deprovisioning is as well as identity provisioning a subfield of Identity and Access Management (IAM). It is an opposite to identity provisioning. While identity provisioning takes care of creating new accounts, determining the roles for individual users and their rights or making changes in them, deprovisioning works oppositely. When an employee leaves the company, his account is deactivated or deleted and he loses all the accesses to both internal and external systems. This way the organization minimizes the information theft and stays secure.
Example
A new manager retires from the organization. Thanks to identity deprovisioning, his company email is deactivated, he loses access to working groups, cloud storage and every other system he had access to.