A century (From the Latin cent, one hundred) is one hundred consecutive years. Latin is an ancient Indo-European language originally spoken in Latium, the region immediately surrounding Rome. ... 100 (the Roman numeral is C for centum) is the natural number following 99 and preceding 101. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ...

• In all dating systems, centuries are essentially numbered ordinally. Thus, the first century of a time frame is "The First Century" and not "Century 0".

• There is considerable disagreement about whether to count the centennial year (e.g. 2000) as the first or last year of a century. This confusion is documented for every centennial year from 1500 onward, and almost certainly arises from the introduction of Hindu-Arabic numerals and the concept of zero to Western Europe in the twelfth century.

The oldest dating systems were regnal, and considered the date as an ordinal, not a cardinal number. Thus, one speaks of the first year of the reign of King John for example. Obviously, the century problem does not arise in such systems. Somewhat later, systems arose dating from the founding of a dynasty, city or religion, and these continued ordinal, rather than cardinal, counting. Thus Ab Urbe Condita counts the Year 1 as the founding of Rome; Anno Domini as the first full year of Jesus Christ's life; the Islamic calendar as the year of the Hejira, so it is also latinized as Anno Hejira. The Hindu-Arabic numeral system (also called Algorism) is a positional decimal numeral system documented from the 9th century. ... 0 (zero) is both a number — or, more precisely, a numeral representing a number — and a numerical digit. ... Ab urbe condita (AUC or a. ... Dionysius Exiguus invented Anno Domini years to date Easter. ... The Islamic calendar or Muslim calendar (Arabic: التقويم الهجري; at-taqwīm al-hijrī; Persian: تقویم هجری قمری Gāhshomāri-ye Hejri; also called the Hijri calendar) is the calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate Islamic... Hijra may refer to: Hijra (Hegira/Hijrah/Hejira) is an Arabic term referring to the migration of the Prophet Muhammad from Mecca to Medina in 622. ...

In the Gregorian calendar, the calendar that is currently used for most purposes nearly everywhere in the world, the first year is that of the traditionally accepted year of Jesus' birth. There is no "year zero". Accordingly, the first century includes the years 1-100 anno Domini, the first millennium (or period of ten centuries) is the years 1-1000 anno Domini, and so on. The Gregorian calendar is the calendar that is used nearly everywhere in the world. ...

More modern systems of dating (such as that of astronomical year numbering, used by astronomers) begin with a year zero. In these cardinal dating systems, it is perfectly logical to use 0 to 99 as the first century, and to regard 2000 as the first year of the twenty-first century. Astronomical year numbering is based on BCE/CE (or BC/AD) year numbering, but follows normal decimal integer numbering more strictly. ... An astronomer or astrophysicist is a scientist whose area of research is astronomy or astrophysics. ...

See also

• Centuries
• Annum
• Millennium