retarder

/rɪˈtɑrdər/
noun
  1. A device or substance that slows down a process or movement.
    • The train uses an electromagnetic retarder to reduce speed without wearing out the brakes.
    • The truck's retarder helped it slow down safely on the steep mountain road.
    • A chemical retarder is added to concrete to prevent it from hardening too quickly.
  2. A person or thing that causes delay or hindrance.
    • In the project, the lack of funding was the main retarder of progress.
    • He was seen as a retarder in the group because he always objected to new ideas.
    • The broken traffic light acted as a retarder, causing long delays.