year 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 period of time containing 365 (or 366) days
“she is 4 years old”
Synonyms, antonyms and the broader and narrower relations for year come from Princeton University’s WordNet. Full sources and licences.
a period of time occupying a regular part of a calendar year that is used for some particular activity
“a school year”
the period of time that it takes for a planet (as, e.g., Earth or Mars) to make a complete revolution around the sun
“a Martian year takes 687 of our days”