deceleration

noun

1. (uncountable)The act or process of decelerating.

   “The rocket is now in deceleration.”

   “Comparing both Regions' 1959 services against those of 1939 an appreciable deceleration is evident. This was due to increased loadings, even though more trains were being run, while on the W.R. in particular the increased volume of freight traffic over the line shared the blame.”

2. (countable)The amount by which a speed or velocity decreases (and so a scalar quantity or a vector quantity), an acceleration having a negative numerical value.

   “The brakes produce a deceleration of 10 metres per second.”