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A groupoid is a category where all morphisms are invertible. This notion can also be seen as an extension of the notion of group. A motivating example is the fundamental groupoid of a topological space with respect to several base points, compared to the usual fundamental group.

6 votes

The groupoid of algebraic expressions and proofs

The construction you describe seems more like the the category of reductions generated by the abstract rewrite system given by an algebraic theory. I suggest you take a look to section 8.2("Rewrite s …
Giorgio Mossa's user avatar
9 votes

Why did Voevodsky consider categories "posets in the next dimension", and groupoids the corr...

In similar way groupoids are next level sets because they are sets, i.e. types with a dependent reflexive, symmetric and transitive type $=$, in which $x=y$ are sets $1$-dimensional objects instead of …
Giorgio Mossa's user avatar