A decade (pronounce: DEK-aid) is a period of 10 years. The word comes from the Ancient Greek word for ten: "deka". 10 decades make a century, while 100 decades make a millennium. A decade is any 10 year period: There are different ways to refer to different groupings of ten years. For example, the period from 2001 to 2010 can be described as the 1st decade of the 21st century. An individual who has been alive two full decades is referred to as being in their 20s for the next decade of their life, from age 20 to 29. Decades are also considered specific groups of ten years sharing the same tens digit, identified by name, as in the nineteen-eighties (1980s) referring to the period from 1980 to 1989 and the nineteen-nineties (1990s) referring to the period from 1990 to 1999. This is the sense meant when someone refers to decades without other context, for example saying 'next decade' in any year between 2020 and 2029 would indicate the 2030s. Sometimes this is shortened to just the decade, where the context is clear, as in the eighties (80s) or the nineties (90s). No apostrophe is used before the s in the names of these decades.
Some decades also have nicknames, such as the Roaring Twenties (1920s), the Gay Nineties (1890s) (US), the Naughty Nineties (1890s) (UK), and the Swinging Sixties (1960s).