noun (n)

• a person who enjoys telling or playing jokes(noun.person)

  Synonym:
  jokester
  

  source: wordnet30
• a person who does something thoughtless or annoying(noun.person)

  Example:
  Some joker is blocking the driveway.
  

  Synonym:
  turkey
  

  source: wordnet30
• an inconspicuous clause in a document or bill that affects its meaning in a way that is not immediately apparent(noun.communication)

  Example:
  When I demanded my money he showed me the joker in the contract.
  

  source: wordnet30
• a playing card that is usually printed with a picture of a jester(noun.artifact)

  source: wordnet30
• One who makes jokes or jests; a humorist; a wag.(noun)

  source: webster1913