A decade is a period of 10 years. The word "decade" originates from the Greek word "dekas," which means a group of 10. The word "decade" began to be used in English to mean a period of 10 years in 1590.
Though a decade refers to any 10-year period, it commonly refers to every 10 years of a century. For instance, the 1920s, the 1960s or the 1980s tend to be identified as defining particular cultures or ways of life. Other words that refer to long periods spanning multiple years are "century" for 100 years and "millennium" for 1,000 years.