Plural form of centenional; a term related to 100-year periods or things occurring every hundred years.
From Latin 'centenionalis,' derived from 'centum' (hundred) + '-onalis' (relating to). This is an archaic or specialized term rarely used in modern English.
This word is so obscure that it barely appears in English texts outside of historical or ecclesiastical documents—it's a perfect example of how specialized Latin-derived terminology can become completely obsolete even while its root words remain common.
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