Definition
An aerodrome to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to or to land at the aerodrome of intended landing. Alternate aerodromes include the following: Take-off alternate — an alternate aerodrome at which an aircraft can land should this become necessary shortly after take-off and it is not possible to use the aerodrome of departure. En-route alternate — an aerodrome at which an aircraft would be able to land after experiencing an abnormal or emergency condition while en route. Destination alternate — an alternate aerodrome to which an aircraft may proceed should it become either impossible or inadvisable to land at the aerodrome of intended landing.
Plain English
A backup airport you can divert to if you can't land where you originally planned. ICAO recognises three types depending on when the diversion might be needed: just after take-off, somewhere along the route, or at the destination.
Context Anchor
You will see this term in flight planning, weather decision-making, dispatch planning, and international or ICAO-based procedures.
Derivation
Aerodrome comes from the Greek 'aer' (air) and 'dromos' (running track or course) — literally 'a course for aircraft.' ICAO and most of the world use 'aerodrome'; the U.S. typically uses 'airport.' They mean the same operational thing in this context.
Why Pilots Care
Ensures a safe landing option exists if weather, runway conditions, or other issues prevent use of the planned airport, and is mandatory for most IFR flight plans.
Intuition Check
Do not read “alternate” as just any nearby airport. In this context, it means a planned backup landing place that is suitable, usable, and expected to be available when needed.
Example Sentence 1
Because the destination forecast showed possible thunderstorms at the arrival time, the crew filed two destination alternate aerodromes on the ICAO flight plan.
Example Sentence 2
Filing an alternate aerodrome is required when the destination forecast shows marginal conditions for the arrival time.