In the manner of or relating to a period of 100 years; occurring or happening every 100 years.
From 'centennial' (relating to 100 years) + '-ly' (adverbial suffix). Follows the standard English pattern of converting adjectives to adverbs describing manner or frequency.
This adverb is almost never used in casual speech because few events actually occur 'centennially'—it's more of a technical term that appears in academic writing about long-term historical cycles or astronomical phenomena.
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