Relating to or of the nature of an accident; occurring by chance or unexpectedly; incidental or non-essential.
From Latin 'accidentalis' (accidental) through French influence. The term is largely archaic, replaced by 'accidental' in modern English, but survives in philosophical and formal contexts.
Medieval philosophers used 'accidentary' to distinguish between essential properties and accidental ones—your substance is unchanging, but your 'accidentary' properties like color or location can change. Deep stuff!
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