A century (from the Latin centum, meaning one hundred; abbreviated c.) is a period of 100 years. Centuries are numbered ordinally in English and many other languages. For example, "the 17th century" strictly refers to the years from 1601 to 1700, but common usage refers it to be 1600 to 1699.
A centenary is a hundredth anniversary or a celebration of this, typically the remembrance of an event which took place a hundred years earlier.
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 of the 19th century).
Dionysius Exiguus of Scythia Minor introduced the anno Domini system in AD 525, counting the years since the birth of Christ. This calendar era is based on the traditionally reckoned year of the conception of Jesus of Nazareth, with AD counting years from the start of this epoch, and BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525, but was not widely used until after 800.
Strictly speaking, centuries begin with years ending with "01" as its last two digits and end with "00". In common usage and popular culture, however, a new century begins with years ending in "00" and end with years ending in "99". For example, strictly speaking, the 20th century runs from 1901-2000, but in common usage the 20th century runs from years 1900-1999. Neither terms however, are incorrect.
|2 BC||1 BC||1||2||3||4||5||...||96||97||98||99||100||101||102||103||...||198||199||200||...||1901||1902||...||1998||1999||2000||2001||2002||2003||...||2098||2099||2100||2101||2102||2103||...||2198||2199||2200|
|1st century||2nd century||...||20th century||21st century||22nd century|
|2 BC||1 BC||1||2||3||4||5||...||96||97||98||99||100||101||102||103||...||198||199||...||1900||1901||1902||...||1998||1999||2000||2001||2002||2003||...||2098||2099||2100||2101||2102||2103||...||2198||2199|
|1st century (99 years only)||2nd century||...||20th century||21st century||22nd century|
There is no "zeroth century" in between the 1st century BC and the 1st century AD. Also, there is no year 0 AD. The Julian calendar "jumps" from 1 BC to 1 AD. The first century BC includes the years 100 BC to 1 BC. Other centuries BC follow the same pattern.
Besides the Gregorian calendar, the Julian calendar, the Aztec calendar, and the Hindu calendar have cycles of years that are used to delineate whole time periods; the Hindu calendar, in particular, summarizes its years into groups of 60, while the Aztec calendar considers groups of 52.
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. Therefore, 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.
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In Swedish, Danish, Norwegian, Icelandic and Finnish, besides the ordinal naming of centuries another system is often used based on the hundreds part of the year, and consequently centuries start at even multiples of 100. For example, Swedish nittonhundratalet (or 1900-talet), Danish nittenhundredetallet (or 1900-tallet), Norwegian nittenhundretallet (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).