Wordnet 3.0
NOUN (2)
1. 
 financial return or reward (especially returns equal to the initial investment); 
2. 
 the act of taking revenge (harming someone in retaliation for something harmful that they have done) especially in the next life; 
- Example: "Vengeance is mine I will repay, saith the Lord"--Romans 12:19; 
- Example: "For vengeance I would do nothing. This nation is too great to look for mere revenge"--James Garfield- Example: "he swore vengeance on the man who betrayed him"- Example: "the swiftness of divine retribution"[syn: vengeance, retribution, payback]
The Collaborative International Dictionary of English v.0.48:
payback \payback\ n.
   same as retribution. [informal]
   [PJC]
   2. same as requital. [informal]
      [PJC]
WordNet (r) 3.0 (2006):
payback
    n 1: financial return or reward (especially returns equal to the
         initial investment)
    2: the act of taking revenge (harming someone in retaliation for
       something harmful that they have done) especially in the next
       life; "Vengeance is mine; I will repay, saith the Lord"--
       Romans 12:19; "For vengeance I would do nothing. This nation
       is too great to look for mere revenge"--James Garfield; "he
       swore vengeance on the man who betrayed him"; "the swiftness
       of divine retribution" [syn: vengeance, retribution,
       payback]