Definitions of joke:

• noun:   a triviality not to be taken seriously

  Example: "I regarded his campaign for mayor as a joke"

• noun:   a humorous anecdote or remark intended to provoke laughter

  Example: "He told a very funny joke"

• noun:   a ludicrous or grotesque act done for fun and amusement
• noun:   activity characterized by good humor
• verb:   tell a joke; speak humorously

  Example: "He often jokes even when he appears serious"

• verb:   act in a funny or teasing way