Webster's explanation of 'century' as a Roman army unit is a little lacking - here it is, in more detail:

A century in a legion of the Roman Republic was half of a maniple - typically, 60 men in the case of the soldiers of the first two lines (the hastati and principes), or 30 of the third-line veterans (the triarii.) In an imperial legion, centuries within the 2nd through tenth cohorts consisted of 80 men, while the centuries of the 1st cohort were double that. A century was the smallest individual fighting unit in the legion.