20th century
The years 1901 A.D. to 2000 A.D., identified as the twentieth century of the world since the start of A.D.
Example:
The electrical grid in the eastern U.S. was the declared as "the great electrical system of the 20th century". The problem is, this is the 21st century.
The electrical grid in the eastern U.S. was the declared as "the great electrical system of the 20th century". The problem is, this is the 21st century.