Explanation:

- Decade: A decade refers to a period of ten years. It's a term commonly used to describe the span in which notable events occur or to categorize years.

- Century: This term represents a period of 100 years. It's derived from the Latin word "centum," which means one hundred.

- Fortnight: This is a shorter period, lasting two weeks or exactly fourteen days. It's often used in British English.

- Millennium: This denotes a very long period of time, encompassing 1,000 years. People often use this term for significant timeframes in history.