How to say "deterioration" in Japanese

• 悪化あっかJLPT N1approx.Common
  noun, suru verb, intransitive verbThe general word for something getting worse: a situation, a medical condition, or relations between parties deteriorate. When unsure which word fits, this is usually the safest choice.
  日ひが経たつにつれて、天気てんきは次第しだいに悪化あっかしつつあった。↗

  The weather was getting worse and worse as the day went on.
• 劣化れっかCommon
  noun, suru verb, intransitive verbCovers deterioration in quality or performance rather than a worsening situation: materials, products, or skills degrading over time. Choose this when the emphasis is on something losing its quality or effectiveness.
• 老廃ろうはいCommon
  noun, suru verb, intransitive verbRestricted to deterioration caused by age: things wearing out or reaching the end of their usefulness. For everyday senses of 'deterioration' this is seldom the word you want; use 悪化 or 劣化 instead.
• 改悪かいあくJLPT N1approx.Common
  noun, suru verb, transitive verbRefers to something being changed for the worse — a revision that makes it worse — rather than natural or gradual deterioration. Unless the worsening stems from a change, use 悪化.
• 低下ていかJLPT N2approx.Common
  noun, suru verb, intransitive verbUsed for a fall or decline in level, degree, or standard — when something drops rather than a situation simply worsening. For plain 'deterioration' of a condition, 悪化 is usually the better fit.
  たしかに出生率しゅっしょうりつの低下ていかで子供こどもの数かずが減へり、学校がっこうでも生徒せいとの数かずが少すくなくなっているのは事実じじつですが、しかし、これはまったくもってけっこうなことです。↗

  It's true that there are fewer children due to the falling birthrate, and there are fewer children in the school, but that is all to the good.

Use 悪化 for a situation or condition getting worse, 劣化 for loss of quality or performance, 低下 for a fall in level or degree, 改悪 for changes that make something worse, and 老廃 only for age-related wearing out.

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

• 悪化あっかdeterioration
• 劣化れっかdeterioration (in quality, performance, etc.)
• 老廃ろうはいdeterioration (due to age)
• 改悪かいあくchanging (something) for the worse
• 低下ていかfall
• 変質へんしつalteration (of character or essence)
• 退廃たいはい(moral) decay
• 老朽化ろうきゅうかdeterioration
• 悪変あくへんdeterioration
• 増悪ぞうあくworsening
• 腐朽ふきゅうdecay
• レベルダウンdecrease in level
• 退歩たいほretrogression
• 廃退はいたい(moral) decay
• 退化たいかregression (e.g. of society)