noun

Definition : an auxiliary rocket engine (as on a spacecraft) used in decelerating due to the friction-less void of space where any and all acceleration remains constant along with inertia. Newton’s First Law – An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Source: www.merriam-webster.com

This has been been used from the start of the space race after a stable rocket design was approved ans there was need to now get it back. Before it was mainly a force on getting a rocket in to high orbit without it failing. Now with maned missions that could go on. Orbit the rules of acceleration change going faster increases altitude while going slower decreases it. this happens naturally due to the pull of gravity from the Earth however that takes an much larger amount of time vs a two minute burn because of the inertia from the force used going into orbit.

I know this word but felt that others may not because retro also means old styles and they may have been thinking of old rockets that people drew in the 60’s.