怪|かいkai[1]atamadaka (head-high)

1. nounmystery; wonder

• Synonym不思議ふしぎNeutralThe everyday word for something wondrous or inexplicable; far more common than 「怪」, which is bookish and mostly appears as a morpheme in words like 「怪事件」「怪人」.
• SynonymミステリーNeutralA loanword for mystery and suspense, and the usual word for detective novels; 「怪」 instead suggests the bizarre and eerie.
• Synonym謎なぞNeutralA riddle or something whose truth is unknown; 「怪」 stresses strangeness rather than an unsolved puzzle.
• Synonym神秘しんぴWrittenA solemn, written-style word for mysteries beyond human understanding; 「怪」 instead carries a suspicious, eerie nuance.
• Related不可解ふかかいWrittenSomething that simply cannot be understood or explained; 「怪」 emphasizes weirdness and suspiciousness.
• Related怪奇かいきWrittenEerie, grotesque strangeness, often a genre label in novels and films; 「怪」 is rarely used alone and mostly forms compounds.
• Related不可思議ふかしぎWrittenA written-style word for the wondrous and hard to explain; 「怪」 carries an eerie, suspicious feel.
• Related怪事かいじWrittenA written-style word for a strange or bizarre incident; 「怪」 itself is more abstract.
• Antonym常つねWrittenThe normal, usual state of things — the written-style opposite of the strangeness 「怪」 conveys.
• AntonymただNeutralOrdinary or commonplace (and also 'free of charge'); as an antonym it contrasts with 「怪」 only in the sense of the ordinary.

For something strange or wondrous, say 「不思議」; for a riddle or unsolved puzzle, 「謎」; for profound mystery, 「神秘」; for the eerie and bizarre, 「怪奇」. 「怪」 is bookish on its own and mostly appears as a morpheme in compounds like 「怪現象」.

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

1. （カイ）あやしいこと。あやしいもの。