Plural form; multiple 100-year anniversaries or multiple people/things relating to 100-year periods.
English plural of 'centennial,' from Latin 'centennium' (period of 100 years) + '-ial' (relating to) + '-s' (plural marker).
Marketers have begun talking about generational 'centennials' to describe every 100-year cohort of people, but the term hasn't caught on because 100 years is too long for people to actually share a common culture or experience.
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