estimation has 4 distinct synonyms spread across 4 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 document appraising the value of something (as for insurance or taxation)
the respect with which a person is held
“they had a high estimation of his ability”
Synonyms, antonyms and the broader and narrower relations for estimation come from Princeton University’s WordNet. Full sources and licences.
an approximate calculation of quantity or degree or worth
“an estimate of what it would cost”
a judgment of the qualities of something or somebody
“many factors are involved in any estimate of human life”