A group, set, or series of ten
"The museum organized a decade of exhibitions to honor the artist's most prolific period."
A period of ten years , particularly such a period beginning with a year ending in 0 and ending with a year ending in 9.
"The fashion trends from the nineties really influenced my style choices back then."
In plain English: A decade is a period of ten years.
"That car has been in my family for over a decade."
Usage: Use "decade" to refer specifically to a period of ten years, not merely any collection of ten items. While the word can technically mean a set of ten in broader contexts, its primary everyday usage denotes a chronological span like the 1990s.
The word "decade" entered English from Old French and Late Latin, where it originally referred to a set of ten items before narrowing specifically to a span of ten years. It was first used in this time-based sense as a shortened form of the phrase "decade of years."