Questions about geometric realization of categories, simplicial objects and any other similar notion of geometric realization.
Geometric realization is the operation that builds from a simplicial set $X$ a topological space $|X|$ obtained by interpreting each element in $X_n$ – each abstract $n$-simplex in $X$ – as one copy of the standard topological $n$-simplex $Δ^n_{Top}$ and then gluing together all these along their boundaries to a big topological space, using the information encoded in the face and degeneracy maps of X on how these simplices are supposed to be stuck together. It generalises the geometric realization of simplicial complexes.