Definition of joke 

1a : something said or done to provoke laughter especially : a brief oral narrative with a climactic humorous twist

b(1) : the humorous or ridiculous element in something

(2) : an instance of jesting : kidding can’t take a joke

c : practical joke

d : laughingstock

2 : something not to be taken seriously : a trifling matter consider his skiing a joke —Harold Callender often used in negative constructions it is no joke to be lost in the desert

(Source: Merriam-Webster)