also: ももとせ
noun
hundred years; century; a very long time
Literally 'one hundred years', also used figuratively for an extremely long time. The archaic reading ももとせ is poetic.
この建物は百年以上前に建てられた。
This building was built over a hundred years ago.
百年に一度の大災害と言われている。
It is said to be a once-in-a-century disaster.
It'll be a hundred years too soon for that day to come.
世紀 specifically means 'century' as a unit of time (e.g., 20世紀 = 20th century), while 百年 is more literal 'one hundred years' and can be used figuratively.
Compound of 百 (hyaku, 'hundred') and 年 (nen, 'year'). The reading ももとせ is an older native Japanese reading of the same kanji.