slow down in WordNet English dictionary

1. lose velocity; move more slowly
   "The car decelerated"
   decelerate  slow  slow down  slow up  retard 
   
2. reduce the speed of
   "He slowed down the car"
   decelerate  slow down 
   
3. become less tense, rest, or take one's ease
   "He relaxed in the hot tub"; "Let's all relax after a hard day's work"
   relax  loosen up  unbend  unwind  decompress  slow down 
   
4. become slow or slower
   "Production slowed"
   slow  slow down  slow up  slack  slacken 
   
5. cause to proceed more slowly
   "The illness slowed him down"
   slow  slow down  slow up 
   

slow down in English Wiktionary