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A decade is a period of ten years. The word is derived (via French and Latin) from the Ancient Greek: δεκάς (/ðɛkˈɑːs/, transliteration=dekas), which means a group of ten. Other words for spans of years also come from Latin: biennium (2 years), triennium (3 years), quadrennium (4 years), lustrum (5 years), century (100 years), millennium (1000 years).

Although any period of ten years is a decade, a convenient and frequently referenced interval is based on the tens digit of a calendar year, as in using "1960s" to represent the decade from 1960 to 1969. Often, for brevity, only the tens part is mentioned (60s or sixties), although this may leave it uncertain which century is meant. These references are frequently used to encapsulate popular culture or other widespread phenomena that dominated such a decade, as in The Great Depression of the 1930s.

Because the common calendar starts with year 1, its first full decade is the years one to ten, the second decade from 11 to 20, and so on. So although the "1960s" comprises the years 1960 to 1969, the "197th decade" spans 1961 to 1970.

A decade may also refer to an arbitrary span of ten years. For example, the statement "during his last decade, Mozart explored chromatic harmony to a degree rare at the time", merely refers to the last ten years of Mozart's life without regard to which calendar years are encompassed.

For decades of the 20th century, the term 'decade' often conjures not just a set of ten years but a distinct era roughly approximating those ten years - for example, the sixties often refer to events that took place between c. 1964 and 1973 and conjure memories of the counterculture, flower power, protests of 1968 and other things going on at the time.