A “century” is more than a hundred years, at least for the Romans
Who doesn’t know that the word "century" mean a long period of hundred years, which now some men or women can reach with joy?. But originally this term had a meaning not so accurate in time. It meant lineage and period in which an entire generation take place until disappear. If the reference is the life of man, it appeared that one hundred years was the maximum term of life and thus acquired this precise numerical value.
The Roman public life was steeped in religious rites, and sometimes they solemnly celebrated the end of a century and the beginning of another. So the Emperor Augustus, who never missed any opportunity to restore and enhance the official cult and publicize their time and work of government, held in the year 17 B.C. the “Secular Games” to announce that something new and great starts. He instructs the poet Horace to do the official anthem of the celebration, Carmen Saeculare, which was recited by choruses of youths and maidens and begins:
O Phoebus, and Diana ruler of the woodlands,
Radiant glory of the sky, O ye who are to be worshiped
Always, and venerated, grant what we pray for
In this sacred season
…
Fostering Sun, thou who in shining chariot the day
Dost reveal and conceal and art as another
Yet the same reborn, may you than the city of Rome be able
To behold nothing greater!
(Translation: Z. Philip Ambrose)
The striking phrase “per saecula saeculorum” (for ever and ever) obviously refers to the endless eternity. In the monastic and religious context, "century" has another meaning, means life "in the world" versus "spiritual life". If "regular" monks are those who follow the rule of the convent, the "secular" clerics are those who live outside in the world.