Definitions
1.NounA period that began on January 1, 1900 and ended on December 31, 1999; almost the same period as the 20th century (which, however, was from year 1901 to 2000)
2.NounA period that began on January 1, 1900 and ended on December 31, 1909; almost the same period as the 1st decade of the 20th century, which, however, was from year 1901 to (the end of) 1910.