according to definition of revenge provided by montresor at the beginning of the story "a wrong is undressed when

I'm not sure what you are asking here. Can you re-phrase your question?

Oh, I see. It goes.. "A wrong is unredressed when retribution overtakes its redresser"

Can you explains it in more details for me please?

Montresor is basically saying that Fortunato has to pay (for his insults)and since he hasn't tried to make things right, he (Montresor) will have to now take matters into his own hands and punish Fortunato himself.

Thanks Aslan!

No problems!