A rider working for their leader: shelters them from the wind, fetches bottles, closes down attacks.
Cycling is a team sport disguised as an individual one. A domestique spends the day at the front of the bunch sheltering their leader, or chasing down breakaways.
The best mountain domestiques (the "lieutenants") are worth their weight in gold: they set the pace on the climbs right up to the point where the leader has to go alone.