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Operads are mathematical devices which describe algebraic structures of many varieties and in various categories. Operads are particularly important in categories with a good notion of homotopy where they play a key role in organizing hierarchies of higher homotopies. Significant examples first appeared in the 1960s, though the formal definition and appropriate generality waited until a decade later. These early occurrences were in algebraic topology in the study of (iterated) loop spaces and their chain algebras.