How to say "arbitrary" in Japanese

• 任意にんいCommon
  no-adjective, na-adjectiveCovers the neutral sense: something left to one's discretion, chosen freely or at random — 'arbitrary' as in 'an arbitrary choice', meaning any one you like. It is neutral, not a criticism.
• 一方的いっぽうてきCommon
  na-adjectiveCovers the negative sense: a decision made one-sidedly, without consulting the people it affects — 'arbitrary' as in 'a unilateral, arbitrary decision'. This is the sense closest to its other meanings 'one-sided' and 'unilateral'.
  この件けんに関かんする彼かれの解釈かいしゃくはあまりにも一方的いっぽうてきだ。↗

  His interpretation of this matter is too one-sided.

Use 任意 for the neutral sense of arbitrary — something chosen at will, discretionary or random — and 一方的 for the negative sense, a decision imposed one-sidedly without regard for the people affected.

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

• 任意にんいoptional
• 一方的いっぽうてきone-sided
• 恣意的しいてきarbitrary
• アービトラリarbitrary
• ほしいままselfish
• 独断的どくだんてきdogmatic