A decade is

• A. a group of ten
• B. a period of ten years
• C. ten months
• D. ten days

Answer: (B)

A decade is a ten-year period. This term is used to describe spans of time in years, such as the decade from 2000 to 2009 or the decades of the 1980s, 1990s, and so on. The key idea is the unit of time: ten years, not ten months or ten days. The phrase a group of ten would refer to ten individuals or objects, not a time span, so it doesn’t fit the common use of the word. Knowing that de- means ten and -cade marks a span of years helps remember that a decade is specifically ten years.