1. a person or thing that hardens.
  2. a substance mixed with paint or other protective covering to make the finish harder or more durable.
  3. Photography. a chemical used to raise the melting point of an emulsion.


  1. a person or thing that hardens
  2. a substance added to paint or varnish to increase durability
  3. an ingredient of certain adhesives and synthetic resins that accelerates or promotes setting

1610s, from harden + -er (1).

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