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:
a) 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;
b) En-route alternate : an aerodrome at which an aircraft would be able to land after experiencing an abnormal or emergency condition while en route;
c) ETOPS en-route alternate: a suitable and appropriate alternate aerodrome at which an aeroplane would be able to land after experiencing an engine shut-down or other abnormal emergency condition while en route in an ETOPS operation.
d) Destination alternate : an alternate aerodrome to which an aircraft may proceed should it become impossible or inadvisable to land at the aerodrome of intended landing.
ICAO (2007) Annex 2, Annex 3, Annex 6/I, Annex 11; Doc. 9713
"The aerodrome from which a flight departs may also be an en-route or a destination alternate aerodrome for that flight." (source: ICAO doc. 9713)