Decade refers to a period of ten years. The word comes from the Ancient Greek "dekas" meaning "a group of ten".
In common usage, decade can refer to:
1. Any ten-year period (e.g. 1990-1999)
2. Specific historical decades (e.g. the Roaring Twenties)
3. In geology, a unit of time equal to ten million years
4. In electronics, a tenfold increase or decrease in frequency