The quality of being essential; the state or condition of being fundamental or necessary (archaic or rare variant of 'essentiality').
From essence + -cy (suffix forming abstract nouns, similar to -ity). This is an older or less common variant of essentiality with Romance language influence.
This word is practically extinct, replaced by 'essentiality' and 'essentialness'—watching words compete and disappear from English teaches us that language is always in silent evolution!
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