急速|きゅうそくkyuusoku[0]heiban (flat)

1. na-adjective, nounrapid (e.g. progress)
   コンピューターは急速きゅうそくな進化しんかを遂とげた。↗

   Computers have made rapid progress.

• Synonym急激きゅうげきNeutralFor changes that are sharp and violent, as in 「急激な変化」; it carries a nuance of intensity, while 「急速」 focuses on fast progress.
• Synonym迅速じんそくNeutralDescribes brisk, efficient handling, as in 「迅速な対応」 — mostly for responses and actions, whereas 「急速」 is mostly for change and development.
• Synonym快速かいそくNeutralEmphasizes very high speed, and on the railway it names the rapid train (「快速電車」); mostly used attributively before nouns.
• SynonymスピーディーNeutralA loanword for brisk, snappy speed, as in 「スピーディーな対応」; common in colloquial speech and advertising.
• Synonym早いはやいNeutralThe everyday adjective for fast or early, and the most common word in speech; 「急速」 leans written and mostly modifies change.
• Related急きゅうNeutralCovers sudden, urgent, or steep, as in 「急に」 and 「急な坂」; the emphasis is on suddenness, while 「急速」 stresses speed.
• Related速やかすみやかNeutralMeans promptly, without delay, as in 「速やかに提出する」; often used in official documents and directives.
• Antonym緩慢かんまんWrittenAntonym: slow and sluggish, as in 「緩慢な動き」; leans toward written prose and stands opposite to 「急速」.
• Antonym徐々じょじょNeutralAntonym: usually as 「徐々に」, meaning gradually, bit by bit — the opposite of the rapid progress 「急速」 describes.

Use 「急速」 for fast change or development. For sharp, violent change pick 「急激」; for quick responses or actions 「迅速」 or 「速やか」; in everyday speech say 「速い」; for gradual change use 「徐々に」.

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

1. 進行しんこうや進展しんてんなどが速すみやかなこと。また、そのようなさま。