不可欠(ふかけつ)

不可欠(ふかけつ) says that something cannot be left out — the thing it describes is a precondition, not merely useful. It is a written and formal word, at home in reports, editorials and business writing rather than in conversation.

Formation: 不(ふ) 'not' + 可(か) 'possible' + 欠(けつ) 'lacking' — literally 'cannot be lacking'.

Grammar

• 〜に不可欠(ふかけつ)だ: is essential for ~ — the thing it is essential *to* takes に
• 不可欠(ふかけつ)な + noun: the everyday attributive form
• 不可欠(ふかけつ)の + noun: a stiffer attributive, common in formal writing

Collocations

• 不可欠(ふかけつ)な要素(ようそ): an essential element
• 不可欠(ふかけつ)な存在(そんざい): an indispensable presence

Register and similar words: in speech, 必要(ひつよう) 'necessary' or 欠(か)かせない 'cannot do without' do the same job more naturally. 必須(ひっす) is equally formal but leans toward things that are required by a rule — required subjects, mandatory fields.