Definition
The standard spacing that air traffic control (ATC) maintains between aircraft operating under Instrument Flight Rules (IFR), using prescribed minimum distances or altitude differences to keep aircraft safely apart when pilots cannot rely on seeing other traffic.
Plain English
The safe gap that controllers keep between aircraft flying on instrument flight plans, since those pilots may be in cloud or otherwise unable to see and avoid each other on their own.
Context Anchor
Seen when an IFR flight is being cleared through, around, or near an active military operations area.
Derivation
IFR stands for Instrument Flight Rules -- the set of rules pilots follow when flying primarily by reference to instruments rather than by looking outside. 'Separation' comes from the Latin separare, meaning 'to set apart.' Together: the act of setting IFR aircraft apart from each other by a safe margin.
Why Pilots Care
Prevents mid-air collisions and terrain conflicts when visibility is too low for pilots to see and avoid traffic themselves.
Intuition Check
Separation does not just mean the aircraft are not touching or not close by guesswork. In ATC use, it means approved spacing that meets a specific rule or standard.
Example Sentence 1
Because military aircraft in an active MOA may be maneuvering unpredictably, ATC cannot always provide IFR separation between them and civilian IFR traffic.
Example Sentence 2
IFR separation standards allow safe operations near active military areas even when clouds prevent visual contact.