Please explain me. I tried the dictionaries but they do not help.
For example ODO gives the following for revenge:
- Inflict hurt or harm on someone for an injury or wrong done to oneself:- I’ll be revenged on the whole pack of you
1.1 [WITH OBJECT] Inflict revenge on behalf of (someone else): - it’s a pity he chose that way to revenge his sister
1.2 Inflict retribution for (a wrong or injury done to oneself or another): - her brother was slain, and she revenged his death
... and it gives this for avenge:
1 Inflict harm in return for (an injury or wrong done to oneself or another):- he vowed in silent fervour to avenge their murders
1.1 Inflict harm in return for an injury or wrong on behalf of (oneself or another): - we must avenge our dead - they avenged themselves on the interlopers
As you can see these two sets of definitions seem interchangable.