Definition
Winds blowing in the same direction the aircraft is traveling, pushing it from behind. A tailwind increases groundspeed (speed over the ground) but does not increase airspeed (speed of the air over the wings). On takeoff and landing, a tailwind lengthens the ground roll and is generally undesirable; in cruise, a tailwind shortens trip time and saves fuel.
Plain English
Wind blowing from behind you, in the direction you're going. It speeds you up over the ground but doesn't help your wings produce lift.
Context Anchor
You will see this term in traffic pattern, runway selection, takeoff and landing, and flight planning discussions.
Derivation
From 'tail' (the rear of the aircraft) and 'wind.' A wind hitting the tail is a wind pushing you forward.
Why Pilots Care
Tailwinds increase groundspeed and can shorten en route time yet lengthen takeoff and landing rolls, directly affecting which runway is chosen in the traffic pattern.
Intuition Check
Tailwinds do not make the airplane fly faster through the air. They make the airplane move faster over the ground.
Example Sentence 1
With a 10-knot tailwind on the runway, the pilot calculated a longer takeoff roll and decided to use the opposite runway instead.
Example Sentence 2
The downwind leg felt fast because of the tailwind pushing the airplane along the pattern.