A decade is a period of 10 year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

s. 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
A lustrum was a term for a five-year period in Ancient Rome.The lustration was originally a sacrifice for expiation and purification offered by one of the censors in the name of the Roman people at the close of the taking of the census...

 (5 years), century
A century is one hundred consecutive years. Centuries are numbered ordinally in English and many other languages .-Start and end in the Gregorian Calendar:...

 (100 years), millennium
A millennium is a period of time equal to one thousand years —from the Latin phrase , thousand, and , year—often but not necessarily related numerically to a particular dating system....

 (1000 years).


Although any period of 10 years is a decade, a convenient and frequently referenced interval is based on the tens digit of the 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
Popular culture
Popular culture is the totality of ideas, perspectives, attitudes, memes, images and other phenomena that are deemed preferred per an informal consensus within the mainstream of a given culture, especially Western culture of the early to mid 20th century and the emerging global mainstream of the...

 or other widespread phenomena that dominated such a decade, as in The Great Depression
Great Depression
The Great Depression was a severe worldwide economic depression in the decade preceding World War II. The timing of the Great Depression varied across nations, but in most countries it started in about 1929 and lasted until the late 1930s or early 1940s...

 of the 1930s

Since the common calendar starts from the year 1
Year 1 was a common year starting on Saturday or Sunday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian calendar...

, its first full decade contained the years from 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.

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