A decade is a period of 10 years. The word is derived (via French and Latin) from the Ancient Greek: δεκάς (//, 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), score (20 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 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 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.
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 1972 and conjure memories of the counterculture, flower power, protests of 1968 and other things going on at the time.
- John H. Conway and Richard K. Guy, The Book of Numbers. New York: Copernicus (1996): 11. ""Score" is related to "share" and comes from the Old Norse "skor" meaning a "notch" or "tally" on a stick used for counting. ... Often people counted in 20s; every 20th notch was larger, and so "score" also came to mean 20."
- "Oxford Dictionaries". askoxford.com. Retrieved 19 January 2015.
- 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