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'''Stack''' is a concept introduced in [[algebraic geometry]] to generalize [[algebraic varieties]] and [[schemes]]. They were originally proposed by [[David Mumford]] on the compactifications of moduli of curves; such stack is now called a [[Deligne-Mumford stack]]. They were later generalized by [[Michael Artin]] to [[Artin stack]]s, or sometimes also called[[algebraic stack]]s. More generally a ''stack'' is a [[category]] acting like a [[moduli space]] with a ''universal family'' parametrizing a family of mathematical objects such as schemes or [[topological space]]s.