What is a period of 10 years called?

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 typically still referred to as being "10") and ends at the end of their 20th year of life, on their 20th birthday; similarly, the third decade of life, when one is in one's twenties or 20s, starts with the 21st year of life, and so on, with subsequent decades of life similarly described by referencing the tens digit of one's age.

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 1960 to 1969 is the 1960s, and the period from 1990 to 1999 is the 1990s. Sometimes, only the tens part is mentioned ('60s or sixties, 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 to the decade immediately preceding AD 10, because there was no year 0.