letter has 4 distinct synonyms spread across 3 of its 8 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 written message addressed to a person or organization
“mailed an indignant letter to the editor”
Synonyms, antonyms and the broader and narrower relations for letter come from Princeton University’s WordNet. Full sources and licences.
owner who lets another person use something (housing usually) for hire
a strictly literal interpretation (as distinct from the intention)
“he followed instructions to the letter”
an award earned by participation in a school sport
“he won letters in three sports”