The goal of this introductory talk on operads will be to give several definitions of this notion as well as its main applications discovered so far. An operad is an universal device which encodes multiple inputs operations and all the ways of composing them. This notion was first used to recognize n-fold loop spaces in algebraic topology (70's). It enjoyed a renaissance in algebra and geometry with the Koszul duality theory and the deformation-quantization of Poisson manifolds respectively (90's). Recently, it was proved to be explicitely connected to Grothendieck-Teichmüller theory (2010's).
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