A century is a period of 100 years, and it is numbered ordinally in English and many other languages. The word century comes from the Latin word "centum," meaning one hundred. Centuries can be used to refer to any arbitrary period of 100 years, but there are two viewpoints on the nature of standard centuries.
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According to the strict construction, the 1st century AD began with AD 1 and ended with AD 100, the 2nd century spanning the years 101 to 200, with the same pattern continuing onward. In this model, the n-th century starts with the year that ends with "01", and ends with the year that ends with "00"; for example, the 20th century comprises the years 1901 to 2000 in strict usage.
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In popular perception and practice, centuries are structured by grouping years based on sharing the hundreds digit(s). In this model, the n-th century starts with the year that ends in "00" and ends with the year ending in "99"; for example, the years 1900 to 1999, in popular culture, constitute the 20th century. (This is similar to the grouping of "0-to-9 decades" which share the tens digit.) .
A centennial or centenary is a hundredth anniversary, or a celebration of this, typically the remembrance of an event which took place a hundred years earlier.
