A group or set of ten things, or the number ten itself.
From Greek 'deca' meaning ten, combined with the suffix '-ad' which denotes a group or collection. The word entered English in the 17th century to describe any collection of ten items or a decade-related concept.
The word 'decad' is rarely used in modern English, but it reveals how different languages try to name the concept of 'ten things' — while we prefer 'decade' or just 'ten,' other cultures and older texts preferred this more systematic grouping term.
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