tailwind

Short definition from Wiktionary

Noun

1. 1
   A wind that blows in the same direction as the course taken by an aircraft, sailing ship, bird, etc.
2. 2
   A force that accelerates progress.

Verb

1. 1
   Of a wind, to blow on a windmill or wind turbine in such a way that wind pressure is exerted on the wrong side of the sail or turbine assembly.