What is a joke?
According to Wiktionary a joke is:
joke (plural jokes)
1 An amusing story.
2 Something said or done for amusement.
"It was a joke!"
4 (figuratively) A worthless thing or person.
"Your effort at cleaning your room is a joke."
"The president was a joke."
Adjectives often applied to "joke": old, bad, inside, poor, silly, funny, lame, hilarious, stupid, offensive.