What does "centuries-old" mean?
A "century" is a period of one hundred years, so "centuries-old" means something is hundreds of years old. "Centuries" is a noun and "old" is an adjective. The hyphen is used to link the separate words of "centuries" and "old" into a single idea that serves as a single word. "Centuries-old" as one word is an adjective. Ex: Trains are a centuries-old mode of transportation. Ex: These books are centuries-old.