Relating to or of the nature of ceremony; ceremonial or formal in character.
From Latin 'caeremonialis' through Old French, this is an older or alternative form of 'ceremonial' that adds the '-ary' suffix typical of Latin-derived English adjectives.
This word is basically the ancestor of 'ceremonial'—it uses the older '-ary' suffix (like in 'ordinary' or 'legendary') instead of the '-ial' suffix, showing how English quietly cycles through different endings as languages evolve.
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