A century is a length of time equal to one hundred years.
The 1st Century was from the Year 1 to the Year 100
The 2nd Century was from 101 to 200
The 20th Century was from 1901 to 2000
The 21st Century is from 2001 to 2100
We are currently in the 21st century.
This is a 5 Dollar gold coin from 1855, which is in the 19th Century