Definition of "deceleration"

deceleration

/diːsɛləˈɹeɪʃən/

noun
countable and uncountable, plural decelerations
1. (uncountable) The act or process of decelerating.
  Quotations
  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.
  1960 February, R. C. Riley, “The London-Birmingham services - Past, Present and Future”, in Trains Illustrated, page 98
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.