A century is a 100 years.
The term can be used in two ways:
- It may apply to any arbitrary period of 100 years, e.g. someone born in 1908 will be a century old in 2008. A century may also be cumulative and non-contiguous, as in a project taking a century of work to complete, even if it was started more than a century ago with pauses in the work.
- A century can also mean one of the repeating 100-year periods moving forward from the Year of Our Lord, e.g. the Twentieth Century A.D. When going backwards in time from Christ, centuries are counted starting with the First Century B.C. and ending with the Forty-First Century B.C. at the Creation of the Universe. As with numerical years, if nothing is appended to the century name, it is assumed that it is A.D.
When a named century is written out in full, as in "the Twenty-First Century", it is considered a proper name and should be capitalized, although it may be referred to more generically uncapitalized, as in "this century".
Because there was no year 0, named centuries begin on years ending in "1", e.g. the Twenty-First Century started on January 1, 2001.