impression has 11 distinct synonyms spread across 6 of its 9 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.
an outward appearance
“he made a good impression”
a clear and telling mental image
“he described his mental picture of his assailant”
a concavity in a surface produced by pressing
“he left the impression of his fingers in the soft mud”
all the copies of a work printed at one time
“they ran off an initial printing of 2000 copies”
(dentistry) an imprint of the teeth and gums in wax or plaster
“the dentist took an impression for use in preparing an inlay”
an impressionistic portrayal of a person
“he did a funny impression of a politician”
the act of pressing one thing on or into the surface of another
“he watched the impression of the seal on the hot wax”
Synonyms, antonyms and the broader and narrower relations for impression come from Princeton University’s WordNet. Full sources and licences.