verb (v)

• make thick or thicker(verb.change)

  Example:
  Thicken the sauce.
  

  Synonym:
  inspissate
  

  source: wordnet30
• become thick or thicker(verb.change)

  Example:
  The sauce thickened.
  

  Antonym:
  thin
  

  source: wordnet30
• make viscous or dense(verb.change)

  Example:
  Thicken the sauce by adding flour.
  

  source: wordnet30
• To make thick (in any sense of the word).(verb)

  source: webster1913
• To become thick.(verb)

  source: webster1913