Plain English Meaning
How long the airplane can stay in the air before it runs out of fuel, flying in the way that uses fuel most slowly.
Full Definition
The maximum length of time an aircraft can remain airborne on a given fuel load, flown at the power setting and configuration that minimizes fuel burn. Endurance is measured in time (hours and minutes), not distance.
Context Anchor
Seen when planning fuel, choosing power settings, and managing aircraft energy during flight.
Derivation
From the Latin 'durare' meaning 'to last' or 'to harden.' Endurance is literally about how long something can last — in this case, how long the airplane can last in the air on its fuel.