Consulting firms call it staff substitution; clients call it contingent labour. Some people say contracting; others, interim management.
The number of terms is an indication of how contentious the issue is and how blurred its definition. What it boils down to is the use of management consultants in roles which should rightly be filled by full-time employees. While it can make sense for an organisation to delay recruiting for the role (the future may be uncertain, the right person hard to find and/or headcount may be limited), hiring an expensive consultant, whose entire training and business model is predicated on finite, usually short-term projects, doesn’t.