Definition
The lowest altitude prescribed by regulation for IFR flight in a given area. It is set to ensure adequate terrain and obstacle clearance and, where required, acceptable navigation signal reception and radio communication coverage. MIAs are established under 14 CFR Part 91 and are reflected on charts as Minimum En Route Altitudes (MEA), Minimum Obstruction Clearance Altitudes (MOCA), Minimum Vectoring Altitudes (MVA), Off-Route Obstruction Clearance Altitudes (OROCA), and similar published minimums.
Plain English
The lowest altitude you are legally allowed to fly at when operating under instrument flight rules in a particular area. It is set high enough to keep you safely above terrain and obstacles.
Context Anchor
Seen in instrument procedure and air traffic control discussions, including Diverse Vector Areas where departure radar vectors must be evaluated against safe minimum altitudes.
Derivation
IFR stands for Instrument Flight Rules. 'Minimum IFR Altitude' simply names the floor below which an IFR flight cannot legally or safely operate in that area.
Why Pilots Care
It guarantees safe obstacle clearance when ATC issues radar vectors instead of published routes.
Intuition Check
Minimum does not mean the lowest altitude that feels comfortable or efficient. Here it means the lowest altitude allowed for instrument flight after obstacle clearance and applicable rules have been considered.
Example Sentence 1
ATC will not clear you below the Minimum IFR Altitude for that sector, even if you request a lower cruise.
Example Sentence 2
The pilot reviewed the MIA before accepting radar vectors to ensure terrain clearance.