急激・急劇|きゅうげきkyuugeki[0]heiban (flat)

1. na-adjectivesudden; abrupt; rapid; sharp; drastic; radical
   その党とうは急激きゅうげきにのびた。↗

   The party gained ground rapidly.

• SynonymにわかWrittenA literary word for things happening all at once, as in 「にわか雨」 or 「にわかに曇る」. 「急激」 centers on how drastic a change is.
• SynonymにわかにWrittenA written adverb for a situation changing all at once, as in 「にわかに曇る」. 「急激」 is a na-adjective that puts more weight on the scale of the change.
• Synonym急にきゅうにNeutralThe most common everyday adverb, covering both suddenness and speed, as in 「急に寒くなる」. 「急激に」 stresses drastic change and leans written/formal.
• Synonym急速きゅうそくNeutralHighlights the speed of progress or development, as in 「急速な発展」. 「急激」 stresses violent, high-impact change; the two are often near-interchangeable.
• SynonymとみにWrittenA literary adverb for a condition deepening markedly over a short time, as in 「とみに衰える」. In speech people say 「急に」 or 「めっきり」.
• Related突然とつぜんNeutralStresses that something happens with no warning; also works as an adverb. 「急激」 focuses on changes being violent and large in scale, as in 「急激な変化」.
• Related不意ふいWrittenA written-style word for something catching you off guard, as in 「不意の来客」. 「急激」 says nothing about surprise; it describes how violent a change is.
• Related突如とつじょWrittenA literary adverb stressing that things appear with no warning at all. 「急激」 captures the violence of a change, e.g. sharp swings in temperature or population.
• RelatedパッとCasualA vivid colloquial mimetic adverb for something flashing open or spreading in an instant, or for quick movements. 「急激」 describes how drastic a change is.
• Related不意にふいにNeutralAn adverb for things happening unexpectedly, catching you off guard. 「急激」 carries no sense of surprise; it means fast, violent change.
• RelatedいきなりCasualA common colloquial adverb for things happening out of nowhere, with no lead-up. 「急激」 stresses violent change or action, not the lack of warning.
• Related出し抜けだしぬけCasualColloquial: doing something before anyone can react, taking them by surprise. 「急激」 has no element of surprise; it describes fast, violent changes or actions.
• Related突発的とっぱつてきWrittenA formal word for events or accidents breaking out unexpectedly, as in 「突発的な事故」. 「急激」 is about the speed and scale of a change.
• RelatedやにわにWrittenA written-style adverb for acting on the spot, right away or without warning. 「急激」 is a na-adjective stressing how violent a change or action is.
• RelatedばっとCasualA colloquial mimetic adverb for a sudden vigorous movement or something spreading at a stroke. 「急激」 describes changes objectively.
• Antonym緩慢かんまんWrittenAntonym in speed of change: movements or changes are slow and sluggish; also used for lax, sloppy handling of matters.
• Antonym漸進的ぜんしんてきWrittenAntonym in manner of change: proceeding step by step toward a goal; often used of reforms and policies.

Use 「急激」 for changes or actions that are sudden and violent in scale, as in 「急激な変化」. When the point is simply fast progress, 「急速」 fits. For lack of warning rather than intensity, use 「突然」, 「急に」 or 「いきなり」; in written prose 「突如」 and 「にわかに」 sound literary. 「不意」, 「不意に」 and 「出し抜け」 add the sense of being caught off guard, which 「急激」 lacks; 「パッと」 and 「ばっと」 are vivid colloquial mimetics for quick movements. The antonyms 「緩慢」 (slow) and 「漸進的」 (gradual) mark the opposite poles of speed and manner.

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

1. 物事ものごとの変化へんか・動うごきが急きゅうで激はげしいこと。