___
___ 世 [period]
is in  ___
On-reading: セイ as in 二世 nisei ___
On-reading: as in 世界 sekai ___
SEI SE (1) age | period (2) world  
二世 nisei 2nd generation japanese-american  
世紀 seiki century 紀 [era] 
十九世紀に jukyūseikini in the 19th century  
中世 chūsei Middle Age  
世界 sekai world | universe  界 [world]