flat has 29 distinct synonyms spread across 13 of its 24 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 3 recorded opposites are contrasty, natural, sharp.
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.
not modified or restricted by reservations
“a categorical denial”
stretched out and lying at full length along the ground
“found himself lying flat on the floor”
lacking contrast or shading between tones
flattened laterally along the whole length (e.g., certain leafstalks or flatfishes)
lacking taste or flavor or tang
“a bland diet”
sounded or spoken in a tone unvarying in pitch
“the owl's faint monotonous hooting”
lacking the expected range or depth; not designed to give an illusion or depth
“a film with two-dimensional characters”
a level tract of land
“the salt flats of Utah”
a shallow box in which seedlings are started
a musical notation indicating one half step lower than the note named
a suite of rooms usually on one floor of an apartment house
5 of the 24 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 flat.
Synonyms, antonyms and the broader and narrower relations for flat come from Princeton University’s WordNet. Full sources and licences.