Learner's definition of DECADE
: a period of 10 years
especially : a 10-year period beginning with a year ending in 0
The war lasted nearly a decade.
a decade of drought [=a drought that lasted 10 years]
The bridge was built a decade ago.
The decade of the 1920s runs from January 1, 1920 to December 31, 1929.
the first decade of the 21st century
There have been a lot of changes during/in/over the past two decades.