imponderable
[adjective]
Synonyms: imperceptible, impalpable, inappreciable, indiscernible, insensible, intangible, unapparent, unappreciable, unobservable, unperceivable

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Antonyms: appreciable, ponderable

[TahlilGaran] English Synonym Dictionary ▲

im‧pon‧der‧a‧ble /ɪmˈpɒndərəbəl $ -ˈpɑːn-/ noun [countable usually plural] formal
something that cannot be exactly measured, judged, or calculated:
There are too many imponderables to make an accurate prediction.
—imponderable adjective:
an imponderable question

[TahlilGaran] Dictionary of Contemporary English ▲