what is decades
decade. 1  a group of ten. 2  a period of ten years; esp., in the Gregorian calendar. 3  officially a ten-year period beginning with the year 1, as 1921-1930, 1931-1940, etc. 4  in common usage, a ten-year period beginning with a year 0, as 1920-1929, 1930-1939, etc.