A period of 100 years reckoned from a certain time, esp. from the beginning of the Christian Era (a.d. 1) [a.d. 1 through a.d. 100 is the first century a.d.; a.d. 1801 through a.d. 1900 is the 19th century a.d.; 400 b.c. through 301 b.c. is the 4th century b.c.]
A unit in ancientRoman, originally of 100 army soldiers as part of a cohort, later of more varied sizes (but typically containing 60 to 70 or 80) soldiers or other men (guards, police, firemen), commanded by a centurion.
A period of 100 consecutive years; often specifically a numbered period with conventional start and end dates, e.g., the twentieth century, which stretches from (strictly) 1901 through 2000, or (informally) 1900 through 1999. The first century AD was from 1 to 100; a yearhundred.
Origin of century
Latin centuriaa group of a hundredfromcentumhundreddekm̥ in Indo-European roots
American Heritage Dictionary of the English Language, 5th Edition