Definition
A wind that blows in the same direction the airplane is moving, pushing it along its path. On takeoff or landing, a tailwind is a wind blowing from behind the airplane down the runway, increasing groundspeed for any given airspeed and lengthening the takeoff or landing roll.
Plain English
Wind coming from behind you, going the same way you are going.
Context Anchor
Seen in takeoff, landing, and performance planning, especially when deciding which runway direction to use.
Derivation
Plain English: 'tail' (the back of the airplane) plus 'wind.' A tailwind is wind hitting the tail — coming from behind.
Why Pilots Care
It lengthens the takeoff and landing roll because ground speed rises while the airspeed needed for lift stays the same.
Intuition Check
Do not think of a tail wind as automatically helpful because it makes the airplane move faster over the ground. For takeoff and landing, that extra ground speed can increase runway needed and reduce safety margin.
Example Sentence 1
With a 10-knot tailwind reported on the runway, the pilot chose to backtrack and depart in the opposite direction into the wind.
Example Sentence 2
Landing with a tail wind meant the airplane crossed the threshold faster over the ground.