Define the nature of revenge in The Odyssey that explains when it is acceptable.

I think that Homer presents a vision of revenge that is contingent on success.  It seems like there is a "might makes right" element present.  Throughout Odysseus' trials, vengeance and anger are presented in the context of animating forces.  If one has the strength or guile to be successful with revenge, so be it.  If one lacks it, then they fall prey to it.  Poseidon has it out for Odysseus.  For the transgressions to his son and his overall sense of hubris, Poseidon wishes nothing but ill will on Odysseus.  It is only because he has curreid favor with Athena that Odysseus is able to withstand Poseidon's challenge.  Regardless of whether Odysseus has it coming, it is a moot point because his support from Athena as well as his ability to demonstrate strength and guile allow him to withstand Poseidon's nature of revenge.  There is little moral order that exists outside of being in favor with the gods and one's own natural abilities.  It is within this light that revenge and all motivations are presented.