不可欠|ふかけつfukaketsu[2]nakadaka (mid-high)

1. na-adjective, nounindispensable; essential
   自由じゆうこそ不可欠ふかけつなものだ。↗

   Freedom is the breath of life.

• Synonym必須ひっすNeutralFor conditions or requirements that must be met, as in 「必須科目」. 「不可欠」 stresses that something falls apart without it.
• Synonym必需ひつじゅNeutralFor things you need no matter what, as in 「生活必需品」 or 「登山の必需品」. 「不可欠」 stresses that without it nothing can hold together.
• Synonym必要ひつようNeutralThe most general word for necessary or needed. 「不可欠」 is stronger, stressing that without it things fail.
• Related重要じゅうようNeutralA broad word for something important or key. 「不可欠」 is stronger, stressing that the thing simply cannot be done without.
• Related大切たいせつNeutralCovers important and precious, often with a feeling of cherishing. 「不可欠」 is the more objective judgment of necessity.
• Related肝要かんようWrittenWritten/formal prose meaning extremely important; in everyday speech 「肝心」 is the usual choice.
• Related緊要きんようWrittenWritten/formal prose for something momentous or pressing, as in 「緊要な課題」.
• Related肝心かんじんNeutralPoints to what matters most at a given moment, as in 「肝心な時に」. 「不可欠」 stresses that something cannot be done without.
• Related枢要すうようWrittenWritten/formal prose for something pivotal or most important, as in 「枢要な地位」.
• Antonym不要ふようNeutralAntonym on the scale of needed versus not needed: it means unnecessary or not required.
• Antonym無用むようNeutralAntonym on the scale of needed versus not needed: it means useless or superfluous.

Use 「不可欠」 for something so essential that without it nothing works or holds together. For general necessity choose 「必要」; for required conditions 「必須」; for must-have items 「必需」; for what matters most 「肝心」. 「重要」 and 「大切」 are broader or more feeling-laden words for important. The antonyms 「不要」 and 「無用」 mean unnecessary or useless.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 無なくてはならない様子ようす。