(せいき)

せいき
noun
century
1. century
A period of 100 years.
二十一(にじゅういち)世紀(せいき)(はい)った
We entered the 21st century.
二十(にじゅっ)世紀(せいき)戦争(せんそう)世紀(せいき)だった
The 20th century was a century of war.
世紀(せいき)大発見(だいはっけん)
The discovery of the century.

世紀(せいき) refers to a 100-year period.

COUNTING CENTURIES:

COMMON EXPRESSIONS:

NOTE ON READINGS:
When counting centuries, 二十世紀(にじゅっせいき) (20th century) uses the reading にじゅっ for 20, with the っ before せ.