nickname has 6 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a familiar name for a person (often a shortened version of a person's given name)
“Joe's mother would not use his nickname and always called him Joseph”
a descriptive name for a place or thing
“the nickname for the U.S. Constitution is `Old Ironsides'”
Synonyms, antonyms and the broader and narrower relations for nickname come from Princeton University’s WordNet. Full sources and licences.