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  "body": "\n## Summary\n\nA groupoid is a [[category|A2DE66937D]] in which every morphism is invertible.\nA [[group|G5S1M9]] is the same as a groupoid with one object, so groupoids generalize groups to symmetries between several objects.\nThe [[fundamental groupoid|ZBHGXJF24E]] of a space, the action groupoid of a [[group action|0WCY42EV9V]] and the groupoid of an [[equivalence relation|0G8VEZ3WTK]] are standard examples.\nGroupoids model homotopy 1-types, and they appear in topology, geometry and the theory of stacks.\n\n## Explanations\n\n### L1 human\n\nA group describes the symmetries of one object.\nA groupoid describes reversible transformations between several objects, where not every pair of objects needs to be connected.\nIn the fundamental groupoid, the objects are the points of a space and the arrows are paths up to homotopy, which can be run backwards.\nUnlike the fundamental group, it does not need a chosen base point.\n",
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