A decade is a period of 10 years (since 1594), a factor of 10 difference between two numbers, or sometimes a set or a group of ten (since 1451). It is also a period of 10 days in the French Republican calendar. The word is derived from the late Latin decas, from Greek dekas, from deka. The other English words for periods of several years also come from Latin: lustrum (5), century (100), millennium (1000).

In English the word can be used to specify a general period of ten years, for example "The last decade was characterized by the rise of the Internet". A decade may also be a well-defined historical period of ten years in a dating system. In that sense, the first decade of the 20th century indicates a period from January 1, 1900 until December 31, 1909.

See also

• List of decades
• Decade nostalgia