A [b]vernier thruster[/b] is a rocket engine used on a [[spacecraft]] for fine adjustments to the direction, attitude or velocity of the vehicle. Depending on the design of a craft's maneuvering and stability systems, it may simply be a smaller thruster complementing the main propulsion system, it may complement larger attitude control thrusters or be a part of the main reaction control system. 

The name is derived from vernier calipers (named after mathematician "Pierre Vernier":https://en.wikipedia.org/wiki/Pierre_Vernier) which have a primary scale for gross measurements and a secondary scale for fine measurements.
バーニアスラスタ