Decade

A decade is a period of 10 years. The word is derived (via French and Latin) from the , which means "a group of ten". Decades may describe any ten-year period, such as those of a person's life, or refer to specific groupings of calendar years. 
Usage.
Any period of ten years is a "decade". 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 Wolfgang Amadeus Mozart's life without regard to which calendar years are encompassed. Also, 'the first decade' of a person's life begins on the day of their birth and ends at the end of their 10th year of life when they have their 10th birthday; the second decade of life starts with their 11th year of life (during which one is usually referred to as being 10) and ends at the end of their 20th year of life on their 20th birthday; the third decade, referred to as being in one's twenties (20s), starts with the 21st year of life (during which one is referred to as being 20) and ends at the end of the 30th year of life on their 30th birthday; subsequent decades of life are described in a similar way by reference to the tens digit of their age. 
0-to-9 decade.
The most widely used method for denominating decades is to group years based on their shared tens digit, from a year ending in a 0 to a year ending in a 9 – for example, the period from 1950 to 1959 is the 1950s, and the period from 1990 to 1999 is the 1990s. Sometimes, only the tens part is mentioned ("50s" or "fifties", and "90s" or "nineties"), although this may leave it ambiguous as to which century is meant. However, this method of grouping decades cannot be applied before AD 10, because there was no year 0.
Particularly in the 20th century, "0-to-9" decades came to be referred to with associated nicknames, 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, referencing the 1890s as the "Gay Nineties" or "Naughty Nineties".
1-to-0 decade.
A rarer approach groups years from the beginning of the AD calendar era to produce successive decades from a year ending in a 1 to a year ending in a 0, with the years 1–10 described as "the 1st decade", years 11–20 "the 2nd decade", and so on; later decades are more usually described as 'the Nth decade of the Mth century' (using the strict interpretation of 'century'). For example, "the second decad of the 12th. Cent." "[sic]"; "The last decade of that century"; "1st decade of the 16th century"; "third decade of the 16th century"; "the first decade of the 18th century". This decade grouping may also be identified explicitly; for example, "1961–1970"; "2001–2010"; "2021–2030". The BC calendar era ended with the year 1 BC and the AD calendar era began the following year, AD 1. There was no year 0.
Public usage of the two methods.
A YouGov poll was conducted on December 2, 2019, asking 13,582 adults in the United States, "When do you think the next decade will begin and end?" Results showed that 64% answered that the next decade would begin on January 1, 2020, and will end on December 31, 2029 ("0-to-9" method); 17% answered that the next decade will begin on January 1, 2021, and will end on December 31, 2030 ("1-to-0" method); 19% replied that they did not know.