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Set | Brilliant Math & Science Wiki
A set is an unordered group of items (called elements). For example, {cat, dog, fish, bird} {cat,dog,fish,bird} is a set of animals, {2, 4, 6, 8, 10} {2,4,6,8,10} is a set of even numbers, and {a, b, c, d} {a,b,c,d} is a set of letters.