connection has 8 distinct synonyms spread across 7 of its 9 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 things or events (as in the case of one causing the other or sharing features with it)
“there was a connection between eating that pickle and having that nightmare”
the state of being connected
“the connection between church and state is inescapable”
an instrumentality that connects
“he soldered the connection”
(usually plural) a person who is influential and to whom you are connected in some way (as by family or friendship)
“he has powerful connections”
the process of bringing ideas or events together in memory or imagination
“conditioning is a form of learning by association”
a supplier (especially of narcotics)
shifting from one form of transportation to another
“the plane was late and he missed his connection in Atlanta”
the act of bringing two things into contact (especially for communication)
“the joining of hands around the table”
Synonyms, antonyms and the broader and narrower relations for connection come from Princeton University’s WordNet. Full sources and licences.