first has 26 distinct synonyms spread across 12 of its 16 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 last, second.
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.
the first element in a countable series
“the first of the month”
the fielding position of the player on a baseball team who is stationed at first of the bases in the infield (counting counterclockwise from home plate)
an honours degree of the highest class
the lowest forward gear ratio in the gear box of a motor vehicle; used to start a car moving
preceding all others in time or space or degree
“the first house on the right”
indicating the beginning unit in a series
highest in pitch or chief among parts or voices or instruments or orchestra sections
“first soprano”
before anything else
“first we must consider the garter snake”
2 of the 16 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for first.
Synonyms, antonyms and the broader and narrower relations for first come from Princeton University’s WordNet. Full sources and licences.