1 a particular period of 100 years that is used for giving dates 世纪:
We live in the 21st century (= the period between the years 2000 and 2099).我们生活在21世纪(2000年至2099年这段时期)
2 any period of 100 years 100年时间:
People have been making wine in this area for centuries.几百年来人们一直在这个地区酿酒。