start has 44 distinct synonyms spread across 20 of its 22 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. Its 2 recorded opposites are stop, finish.
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.
set in motion, cause to start
“The U.S. started a war in the Middle East”
begin work or acting in a certain capacity, office or job
“Take up a position”
the act of starting something
“he was responsible for the beginning of negotiations”
a line indicating the location of the start of a race or a game
a signal to begin (as in a race)
“the starting signal was a green light”
the advantage gained by beginning early (as in a race)
“with an hour's start he will be hard to catch”
Synonyms, antonyms and the broader and narrower relations for start come from Princeton University’s WordNet. Full sources and licences.