[PL 431] Revenge is sweet

The Merriam-Webster dictionary gives the following definitions for revenge:

1. A desire for revenge.
2. An act or instance of retaliating in order to get even.
3. An opportunity for getting satisfaction.

Given these definitions, a few questions arise:

1. If one has the desire for revenge, but does not act on that desire, is that still revenge?
2. What does it mean "to get even"? "Even" in terms of what?
3. If one thinks wrongly that one has been wronged, and seeks retaliation in order to get even, is that revenge? In other words, if the end result is not "even," is that still revenge?
4. If one does not derive any satisfaction from an act of retaliation is that still revenge?

Under what circumstances, if any, is revenge morally justifiable?