A decade is a period of 10 years. The word is derived (via French) from the Ancient Greek dekas which means ten. This etymology is sometime confused with the Latin decas (ten) and dies (days), which is not correct. The other words for spans of years come from Latin: lustrum (5 years), century (100 years), millennium (1000 years).
Although any period of 10 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 1 to 10, the second decade from 11 to 20, and so on. So while the "2000s" comprises the years 2000 to 2009, the "201st decade" spans 2001 to 2010.
A decade may also refer to an arbitrary span of 10 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 10 years of Mozart's life without regard to which calendar years are encompassed.
Thus, an unqualified reference to, for example, "the decade" or "this decade" may have multiple interpretations depending on the context.
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. 1963 and 1971 and conjure memories of the counterculture, flower power and other things going on at the time.
See also 
- Larousse, etymology of decade(French)
- Oxford dictionary definition of "decade"
- Webster dictionary definition of "decade"
- The OWL at Purdue: Apostrophe
- "1960s". Memidex/Wordnet Dictionary/Thesaurus. Retrieved 2011-08-18.
- Passim, i.a. Spencer, Donald D. 1989. Invitation to number theory with Pascal. Ormond Beach: Camelot. 46: "The first decade is from 1 to 10 inclusive, the second decade from 11 to 20 inclusive, and so on."
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- Definition from Etymology Online