A century (from the Latin centum, meaning one hundred) is one hundred consecutive years. It is also a Roman term. Centuries are numbered ordinally in English and many other languages (e.g. "the seventh century AD/CE").
Start and end in the Gregorian Calendar
According to the Gregorian calendar, the 1st century AD/CE started on January 1, 1 and ended on December 31, 100. The 2nd century started at year 101, the 3rd at 201, etc. The n-th century started/will start on the year (100×n)-99 and ends in 100×n . A century will only include one year, the centennial year, that starts with the century's number (e.g. 1900 is the final year in the 19th century).
1st century BC and AD
There is no "zeroth century" in between the 1st century BC and the 1st century AD . Also, there is no 0 AD. The Julian calendar "jumps" from 1 BC to 1 AD. The 1st century BC includes the years 100 BC to 1 BC. Other centuries BC follow the same pattern.
Dating units in other calendar systems
Besides the Gregorian calendar, the Julian calendar and the Hindu calendar have cycles of years which are used to delineate whole time periods; the Hindu calendar, in particular, summarizes its years into groups of 60.
Centuries in astronomical year numbering
Astronomical year numbering, used by astronomers, includes a year zero (0). Consequently, the 1st century in these calendars may designate the years 0 to 99 as the 1st century, years 100 to 199 as the second etc. However, in order to regard 2000 as the first year of the 21st century according to the astronomical year numbering, the astronomical year 0 has to correspond to the Gregorian year 1 BC.
Alternative naming systems
In Swedish, Danish, Norwegian and Finnish, centuries are typically not named ordinally, but according to the hundreds part of the year, and consequently centuries start at even multiples of 100. For example, Swedish nittonhundratalet (or 1900-talet), Danish and Norwegian nittenhundredetallet (or 1900-tallet) and Finnish tuhatyhdeksänsataaluku (or 1900-luku) refer unambiguously to the years 1900–1999. The same system is used informally in English. For example, the years 1900–1999 are sometimes referred to as the nineteen hundreds (1900s). This is similar to the English decade names (1980s, meaning the years 1980–1989).
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