核|かくkaku[1]atamadaka (head-high)

1. nounstone (of a fruit); pit; pip
2. nouncore (of an organization, team, etc.); nucleus; heart
3. no-adjectivenuclear weapons
   この核かく時代じだいにあっては、世界中せかいじゅうどこの国くにだって安全あんぜんではない。↗

   No country under the sun is safe in this nuclear age.
4. noun, physicsnucleus (of an atom)
5. noun, biologynucleus (of a cell)
6. noun, meteorologycondensation nucleus
7. noun, astronomy(planetary) core
8. noun, chemistryring (in a cyclic compound)
9. noun, mathkernel; core
10. nounnucleus (of a cultured pearl)

• Synonym中心ちゅうしんNeutralThe most versatile: the positional center, also the center of talk or activity, as in 「町の中心」「話題の中心」. 「核」 leans toward the inner backbone or kernel, not a location.
• Synonym中枢ちゅうすうWrittenWritten/formal prose. A nerve center or the hub of an institution, as in 「中枢神経」; the stress falls on the central part that directs and controls.
• Synonym核心かくしんNeutralThe crux of a problem or matter, as in 「核心に迫る」; mostly abstract. 「核」 also covers the abstract core but adds concrete senses like fruit stones and nuclei.
• SynonymコアNeutralLoanword for a core, or as a modifier meaning hardcore, as in 「コアなファン」; colloquial and very common.
• Synonym骨髄こつずいWrittenWritten/formal prose. Bone marrow, or by extension the depths of the heart, as in 「恨み骨髄に徹す」; physiological or figurative, not organizational core.
• Synonym本質ほんしつWrittenWritten/formal prose. The essence or true nature of something, as in 「問題の本質」; an abstract property, not a central part or nucleus.
• Synonym骨子こっしWrittenWritten/formal prose. The skeleton or essentials of a plan or bill; used for the content of documents, not a physical or organizational core.
• Synonym要領ようりょうNeutralThe gist of something, or the knack of doing it; about points and methods, not the concrete core that 「核」 names.
• SynonymかなめNeutralThe linchpin, like a fan's pivot: the person or thing that holds everything up, as in 「チームの要」「守備の要」; 「核」 stresses the inner backbone.
• Synonym基幹きかんWrittenWritten/formal prose. The backbone that underpins a system, as in 「基幹産業」; the trunk of industries or systems rather than a nucleus.
• Synonym中軸ちゅうじくWrittenWritten/formal prose. The central axis or the key figure at it, as in 「打線の中軸」; it leans toward the key people at the center of a lineup or group.
• Synonym芯しんNeutralA core that is usually concrete: pencil lead, the core of a fruit, or deep in the body as in 「芯まで冷える」; it leans toward the physical inner part of a thing.
• Synonym要点ようてんNeutralThe main points of what is said or written; about content, not the core of a thing or group.
• Synonym眼目がんもくWrittenWritten/formal prose. The main point or focus of something; leans toward purpose or aim rather than a central part.
• Synonym枢軸すうじくWrittenWritten/formal prose. A center of power or the WWII Axis powers; political and historical uses, not fruit stones or nuclei.
• SynonymエッセンスNeutralLoanword for essence; the distilled element of something, not a physical core or a central member.
• SynonymポイントNeutralLoanword for the key point of a talk or text; colloquial and very common, with none of 「核」's concrete senses.
• Synonym原子核げんしかくNeutralThe atomic nucleus, a physics term; 「核」 can serve as its short form.
• Synonym心しんNeutralRead 「しん」, it means the center or core, as in 「心棒」; points to the innermost center, not fruit stones or weapons.
• Synonym心臓部しんぞうぶWrittenWritten/formal prose. The vital part of a machine or organization; a metaphorical heart, not the literal stone of a fruit.
• Synonym主意しゅいWrittenWritten/formal prose. The central meaning or main idea, especially of a text; about its thesis, not a core part or nucleus.
• Synonym髄ずいWrittenWritten/formal prose. Marrow or pith, the soft tissue inside bones and stems; a biological term, not the core of a team or issue.
• Synonym心髄しんずいWrittenWritten/formal prose. The essence or innermost core of something; rare in modern Japanese.
• Related重心じゅうしんNeutralThe physical center of gravity or balance point, as in 「重心を低くする」; it leans toward mechanical balance, while 「核」 names an inner core or kernel.
• Related真ん中まんなかNeutralThe exact middle, as in 「部屋の真ん中」; purely positional, with none of 「核」's sense of a backbone or kernel.
• Related枢要すうようWrittenWritten/formal prose. Means most important or pivotal, as in 「枢要な地位」; describes a position's status rather than naming a core.
• Antonym殻からNeutralThe hard shell wrapping the outside — the opposite of the 「核」 inside.
• Antonym皮かわNeutralThe outer skin or peel; the opposite of the inner core that 「核」 refers to.

「核」 covers fruit stones, atomic nuclei, nuclear weapons, and the core of a team or discussion, as in 「チームの核」. For the abstract crux of a problem, 「核心」 is the usual choice; for a positional center, use 「中心」 or 「真ん中」. The written terms 「中枢」, 「基幹」, 「中軸」 and 「心臓部」 fit institutions, systems and machines, while 「骨子」, 「要点」 and 「ポイント」 handle the points of texts and talks. For physical cores pick 「芯」 or 「コア」, and remember the opposites 「殻」 and 「皮」 for the outer shell and skin.

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

1. ものごとの中心ちゅうしんとなるもの。
2. 原子核げんしかく。
3. 相あい転移てんいの開始かいし点てんとなるもの。結晶化けっしょうかの場合ばあいは結晶けっしょう核かくという。
4. 環式かんしき有機ゆうき化合物かごうぶつの骨格こっかく部分ぶぶん。ベンゼン環かん（ベンゼン核かく）など。
5. 真核しんかく細胞さいぼうの細胞核さいぼうかく。
6. 内ない果皮かひが硬化こうかし、種子しゅし本体ほんたいを保護ほごするようになったもの。「さね」とも。
7. 天体てんたいの中心ちゅうしん核かく。
8. 真珠しんじゅの養殖ようしょくで、母貝ぼがいに入いれる球たま。
9. 核兵器かくへいき。
10. ベクトル空間くうかん V と W、線型せんけい写像しゃぞう f: V → W、ゼロベクトル 0 について、{v ∈ V | f(v) = 0} で定義ていぎされる V の部分ぶぶん空間くうかん。身近みじかな例れいとしては、右辺うへんを全すべて零れいとする連立れんりつ一いち次じ方程式ほうていしき Ax = 0 を満みたす全すべての解かい。
11. 群ぐん G と G′、準じゅん同型どうけい写像しゃぞう f: G → G′、単位たんい元もと e について、{a ∈ G | f(a) = e} で定義ていぎされる G の正規せいき部分ぶぶん群ぐん。
12. 《日本にっぽん語学ごがく》アクセント核かく。