noun
century; one century
Refers to a period of 100 years, often used in historical contexts or to specify a particular century (e.g., 20世紀).
この建物は一世紀以上前に建てられました。
This building was built over a century ago.
一世紀は百年に当たる。
One century is equivalent to one hundred years.
世紀 means 'century' in general, while 一世紀 specifically means 'one century' or 'a century'.
Compound of 一 (いち, 'one') + 世紀 (せいき, 'century'), with the reading いち changing to いっ before せ due to regular sound change.