熱心|ねっしんnesshin[1]atamadaka (head-high)

1. na-adjective, nounzealous; enthusiastic; ardent; fervent; keen; eager
   この本ほんは彼かれの熱心ねっしんな研究けんきゅうの成果せいかである。↗

   This book is a result of his enthusiastic research.

• Synonym熱意ねついNeutralA noun for enthusiasm and drive itself, as in 「熱意を持つ」; 「熱心」 describes the attitude instead.
• Synonym熱情ねつじょうWrittenWritten/formal prose. A noun for burning, blazing passion; more emotional than 「熱意」.
• Synonym懸命けんめいWrittenWritten/formal prose. Stresses exerting every last bit of effort; usually as 「懸命に」 or 「懸命な努力」.
• Synonym情熱じょうねつNeutralA noun for intense, burning passion, as in 「情熱を注ぐ」; carries stronger emotion than 「熱意」.
• Synonym一途いちずNeutralStresses focusing on one thing or one person to the exclusion of all else; often describes single-minded devotion in love.
• Synonym熱誠ねっせいWrittenWritten/formal prose. Stresses sincere, heartfelt earnestness, as in 「熱誠を込めて」; rare in everyday use.
• Related熱烈ねつれつNeutralEmphasizes intense, outwardly shown emotion; typically modifies welcomes or support, as in 「熱烈な歓迎」.
• Related真剣しんけんNeutralStresses being serious and not joking; does not necessarily imply enthusiasm. Also means a real (not wooden practice) sword.
• Related本気ほんきCasualColloquial. Stresses that you mean it rather than joke, as in 「本気で言う」.
• Related真摯しんしWrittenWritten/formal prose. Stresses a sincere, earnest attitude, as in 「真摯に受け止める」.
• Related一途いちずNeutralStresses focusing on one thing or one person to the exclusion of all else; often describes single-minded devotion in love.
• Related熱狂ねっきょうNeutralFrenzied excitement far more intense than 「熱心」; can also work as a verb, 「熱狂する」.
• Related熱血ねっけつNeutralHot-blooded passion; mostly seen in set phrases like 「熱血漢」 or 「熱血教師」.
• Related献身的けんしんてきWrittenWritten/formal prose. Stresses sacrificing yourself in devoted service to a person or cause.
• Related一生懸命いっしょうけんめいNeutralStresses striving with all one's might; also works as an adverb, and is very common in daily life.
• Related一筋ひとすじNeutralDevotion to a single pursuit, as in 「仕事一筋」; usually follows a noun. Also has the literal sense of one strand or line.
• Related躍起やっきNeutralFrantic, overeager effort, usually as 「躍起になる」; carries an impatient, worked-up nuance.
• Related熱中ねっちゅうNeutralVerbal: being absorbed or engrossed in something, as in 「ゲームに熱中する」.
• RelatedひたすらWrittenWritten/formal prose. An adverb for doing one thing single-mindedly; today usually written 「ひたすら」 in kana, the kanji 「只管」 being old-fashioned.
• Related意欲いよくNeutralA noun for the willingness or eagerness to do something, as in 「意欲がある」.
• Related熱狂的ねっきょうてきNeutralDescribes frenzied, almost uncontrolled enthusiasm; mostly used of fans and audiences.
• Antonym無関心むかんしんNeutralAntonym: shows no interest at all, the opposite of the engaged attitude of 「熱心」.
• Antonym冷淡れいたんNeutralAntonym: a cold, unenthusiastic attitude; can also describe treating people heartlessly.

Use 「熱心」 to describe someone devoted to an activity. For all-out effort, choose 「一生懸命」 or 「懸命」. For enthusiasm as a noun, use 「熱意」 (drive) or 「情熱」 (stronger passion). Say 「熱中する」 for being absorbed in something. 「熱烈」 fits intense welcomes or support, 「熱狂的」 frenzied fans, 「一途」 single-minded love, 「本気」 being serious rather than joking, and 「無関心」「冷淡」 the opposite.

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

1. ある物事ものごとに懸命けんめいに打ち込うちこむ様子ようす。