Alternative monickers that we refer to others as. Usually they're names or a descriptor used as a name that reflects said person in some way- or might relate to some sort of thing they are known for although sometimes not always agreeable with the person in question. They are often applied with fondness when between friends and family.

For example, my father calls myself and my brothers all by colors. I'm Purple, Leo's Blue... Amongst my brethren we all have more than a few ways we call each other. Sometimes it's just a shorter way of saying someone's name. "Donnie" for instance, is a nickname. Short for "Donatello."