Century: definition, history, naming and related terms

Origin and linguistic roots

The English word century derives from the Latin root centum, meaning "one hundred"; the Latin term and its derivatives appear in many modern languages. The Roman numeral for one hundred is "C". In ancient Rome the related word centuria described a military grouping organized around roughly a hundred men; this Roman usage is discussed in historical sources and summaries of Roman military structure — see ancient Rome and the unit concept in centuria descriptions and unit organization.

How centuries are numbered and dated

In calendar practice the first century of the Common Era covers years 1 through 100; because the Gregorian and Julian calendars do not include a year zero, the nth century covers years (100×(n-1)+1) through (100×n). This leads to the convention that the 20th century comprises 1901–2000, while the 21st century began in 2001. In everyday speech people sometimes use century names loosely to mean the set of years that share the same hundreds digits (for example, "the 1900s"), so it helps to be explicit when precise dating matters.

Anniversaries and adjectival forms

Centuries are commonly celebrated or commemorated. A centenary or centennial marks the 100th anniversary of an event; usage varies by region, with "centenary" more common in British English and "centennial" frequent in American English. Longer anniversaries follow the same pattern: bicentenary or bicentennial for 200 years, tercentenary or tercentennial for 300 years. The adjectival form "centennial" also describes something that lasts 100 years or recurs every hundred years.

Related terms and social significance

Closely related words include centum (the Latin for one hundred) and centenarian, a noun for a person who has reached the age of 100 years. The number of centenarians has grown in many parts of the world as life expectancy has increased, making the term increasingly relevant in demographics, health policy and social planning.

Quick reference

• Length: 100 years.
• Roman numeral: C.
• Roman military: centuria originally denoted a unit of roughly one hundred soldiers.
• Anniversary terms: centenary/centennial, bicentenary/bicentennial, tercentenary/tercentennial.
• Calendar numbering: the 1st century = years 1–100; the 21st century began in 2001.