verb

• (transitive) To reduce the velocity of something.
  "In May, for example, the Illinois Central RR. introduced the long-expected streamline version of the Panama Limited, with diesel haulage, cutting the time over the 921 miles between Chicago and New Orleans by 2hr. to 18hr. each way—the fastest on record—but decelerated other trains over the same route."
• (transitive) To reduce the rate of advancement of something, such as a disease.
• (intransitive) To go slower.
  "One of the disadvantages of increasing intermediate stops is that, as we have pointed out already, certain prestige trains have to suffer, notably the "Cornish Riviera Express", slowed 15min to Plymouth by stops at Taunton and Exeter; the "Torbay Express", decelerated 18min by stops at Reading, Westbury, Taunton and Newton Abbot; and the "Bristolian", which adds 14min in order to allow for a Bath stop and an increased load."