果断|かだんkadan[0]heiban (flat)

1. na-adjective, noundecisive; resolute; drastic

• Synonym断固だんこNeutralStresses an unyielding attitude rather than quick decision; works adverbially as in 「断固反対」 and in 「断固たる決意」.
• Synonym果敢かかんNeutralActing decisively and boldly to take on a challenge; very close to 「果断」 in sense, and more common than it.
• Related大胆だいたんNeutralAbout sheer boldness and daring to act; implies courage, not necessarily deciding quickly.
• Related勇敢ゆうかんNeutralHighlights courage in the face of danger; it says nothing about how quickly one decides.
• Related不敵ふてきNeutralFearless, often with a cocky or defiant nuance as in 「不敵な笑み」; has nothing to do with decisiveness.
• Related強硬きょうこうNeutralHard-line and uncompromising in stance; typical of diplomacy and negotiations, as in 「強硬姿勢」.
• Related剛毅ごうきWrittenA written, formal word for strength of will and fortitude; describes inner character, not decisiveness in action.
• Related勇猛ゆうもうWrittenA written, formal word for valor; typically describes warriors and soldiers, stressing bravery rather than decisiveness.
• Related豪胆ごうたんWrittenA written, formal word for having nerve and staying unshaken; stresses boldness rather than decisiveness.
• Related大胆不敵だいたんふてきNeutralFour-character word for reckless fearlessness, often implying audacity or disregard for others; about boldness, not decision-making.
• Related揺るぎないゆるぎないNeutralAn adjective for something unshakeable; stresses firmly held conviction, as in 「揺るぎない自信」.
• Antonym優柔不断ゆうじゅうふだんNeutralIndecisive and wavering, unable to make up one's mind; the direct opposite of 「果断」, which decides firmly and without hesitating.

「果断」 is a fairly bookish word for deciding firmly without hesitation, met mostly in written phrases like 「果断な処置」; in plain speech 「思い切りがいい」 says much the same. Use 「果敢」 for boldly taking on a challenge and 「断固」 for a firm, unyielding stance; 「優柔不断」 is the opposite. The rest stress courage or firm conviction rather than decisiveness.

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

1. 思い切おもいきって物事ものごとを行おこなうこと。