relationship has 3 distinct synonyms spread across 2 of its 4 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 relation between people; (`relationship' is often used where `relation' would serve, as in `the relationship between inflation and unemployment', but the preferred usage of `relationship' is for human relations or states of relatedness)
“the relationship between mothers and their children”
a state of connectedness between people (especially an emotional connection)
“he didn't want his wife to know of the relationship”
Synonyms, antonyms and the broader and narrower relations for relationship come from Princeton University’s WordNet. Full sources and licences.
a state involving mutual dealings between people or parties or countries
(anthropology) relatedness or connection by blood or marriage or adoption