Definition
TRAFFIC MANAGEMENT INITIATIVE (TMI)
Tools used to manage demand with capacity in the National Airspace System (NAS.) TMIs can be used to manage NAS resources (e.g., airports, sectors, airspace) or to increase the efficiency of the operation. TMIs can be either tactical (i.e., short term) or strategic (i.e., long term), depending on the type of TMI and the operational need.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).