verb (v)

• make sweeter in taste(verb.perception)

  Synonym:
  dulcify, dulcorate, edulcorate
  Antonym:
  acetify, acidify, acidulate, sour
  

  source: wordnet30
• make sweeter, more pleasant, or more agreeable(verb.change)

  Example:
  Sweeten a deal.
  

  source: wordnet30
• To make sweet to the taste; as, to sweeten tea.(verb)

  source: webster1913
• To become sweet.(verb)

  source: webster1913