Definition
A specified altitude in a precision instrument approach at which the pilot must decide to either continue the approach to landing or execute a missed approach. The decision must be made when the aircraft reaches this altitude, based on whether the required visual references for the runway are in sight. Decision Altitude is referenced to mean sea level (MSL) and is shown on the approach chart.
Plain English
The exact height above sea level during an instrument approach where the pilot must look up and decide: do I see the runway clearly enough to land, or do I go around? There is no waiting and no descending below it without making that call.
Context Anchor
Seen on instrument approach charts, especially for ILS and other approaches that guide the aircraft down toward the runway.
Derivation
Decision Altitude is named for exactly what happens there: the altitude at which a binding decision must be made. The word 'altitude' (from Latin altus, 'high') signals that this value is measured from sea level, not from the ground -- which distinguishes it from Decision Height, a related but different reference.
Why Pilots Care
Prevents descent below a safe height without visual confirmation of the runway, reducing risk of controlled flight into terrain.
Intuition Check
Do not read Decision Altitude as a place to pause and think it over. It is the pre-set altitude where the decision must already be made: continue visually or immediately climb away.
Example Sentence 1
Approaching the published Decision Altitude on the ILS, the captain called 'minimums' and, with the runway in sight, continued to landing.
Example Sentence 2
The approach plate lists decision altitude as 450 feet MSL for this ILS.