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. Decades in the 20th century are normally called "the eighties" or "the nineties", and written "the 80s" or "the 90s". Decades in other centuries are written in full, e.g. "the 1890s".