Someone operating alone, unaligned or against alliance orders, attacking other people with little regard for the own survuval. Often it is an older player who is leaving the game and who wants to throw around some nukes on the way out.
Rogues like it, the rest of the world does not. Rogues also add to the global radiation, by using their own nukes, and by being deemed so dangerously irresponsible that they must be nuked themselves as fast and as often as possible in order to neutralize them.
Someone who uses an alliance affiliation without being a recognized member of that alliance. Some ghosts do this to escape attacks, they pretend to belong to a big strong alliance. Some ghosts are rogues, they hope to cause trouble for the alliance they are ghosting while nuking around. Some ghosts are newbies who do not know that you have to formally apply for membership first. Alliances will usually try to get rid of ghosts fast, but some alliances are so big that ghosts will not be noticed for a while.
ZI = zero infrastructure, attacking a nation until its infrastructure is close to zero. I don't know about N-ZI. P-ZI is permanent ZI, taking someone down to zero and then keep them there forever by constant war. It is the closest thing to "killing" someone in the game, because often such nations will eventually give up and leave.
Nations can be sanctioned by the senators of the team colours. Rogues will often be sanctioned. If a nation is sanctioned on for example the blue team, then that nation cannot trade with other blue nations, and/or it cannot exchange aid with such nations.
Attacking someone for the purpose of stealing their technology, land and money.
I think you can form your own opinion.
Edited to add: Uh, I see your nation bio states you are "former Spartan leader". If these were joke questions, then congrats. If you really did not know, then there is nothing wrong with that, it is just a bit peculiar.