A decade is a period of 10 years. The word is derived (via French and Latin) from the Ancient Greek: δεκάς, romanized: 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).
- Any period of ten years is a decade, including any arbitrary span of ten years; for example, the statement that "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.
- A popular and frequently referenced interval is based on the tens digit of a calendar year, as in using "the 1960s" to represent the decade from 1960 to 1969. Sometimes, 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.
- However, the Gregorian calendar begins with the year AD 1. (There was no year "zero", and the year before AD 1 is 1 BC with nothing in between.) Therefore, the first decade is from AD 1 to AD 10, the second decade from AD 11 to AD 20, and so on. So, although "the 1960s" comprises the years 1960 to 1969, the years 1961 to 1970 comprise "the 197th decade" in the Gregorian calendar.
- Particularly in the 20th century, a nominal decade is often used to refer not just to a set of ten years but rather to a period of about ten years - for example, the phrase the sixties often refers to events that took place between around 1964 and 1972, and to memories of the counterculture, flower power, protests of 1968 and other things happening at the time. Often, such a nominal decade will come to be known by a title, such as the "Swinging Sixties" (1960s), the "Warring Forties" (1940s) and the "Roaring Twenties" (1920s). This practice is occasionally also applied to decades of earlier centuries, for example, references to the 1890s as the "Gay Nineties" or "Naughty Nineties".