Interim Management is the provision of experienced, typically senior talent on short notice to manage some kind of corporate priority when nobody internally is suitable or available.

Wikipedia provides a solid definition of interim management.

Although the term interim management is commonly used, companies may also refer to it as contract or temporary management. Frequently these terms refer more to gap-filling (sick leave, etc.) than leading change or a large initiative within an organization.

Typically, interim managers are executives or managers with previous, credible experience as leaders in organizations.  Some turn to interim management as a career choice. Others use it to bridge between corporate positions.