For a topological group acting on a topological space , its Borel construction or Borel space is another topological space , also known as the homotopy quotient. In many cases, its ordinary cohomology is the -equivariant cohomology of .
For a topological space, a topological group and a continuous -action, the Borel construction of is the topological space , hence quotient of the product of with the total space of the -universal principal bundle by the diagonal action of on both.
Observing that itself as a groupoid has the nerve
(where ”” denotes the multiplication action of on itself) and regarding and as topological 0-groupoids ( as a group object in topological 0-groupoids), hence with simplicially constant nerves, we have an isomorphism of simplicial topological spaces
If this is set up in a sufficiently nice category of topological spaces, then, by the discussion at geometric realization of simplicial topological spaces, the geometric realization manifestly takes this to the Borel construction (since, by the discussion there, it preserves the product and the quotient).
If is the topological category associated to the group , then a -space is precisely a Top-enriched functor in a similar fashion to the fact that an R-module is an Ab-enriched functor. If is a -space, the ordinary quotient is the colimit of the diagram associated to and the Borel construction is (a model of) the homotopy colimit of that diagram. This is a reason for calling the Borel construction homotopy quotient in some contexts.
The nature of the Borel construction as the geometric realization of the action groupoid is mentioned for instance in
Alejandro Adem, Michele Klaus, Lectures on orbifolds and group cohomology (pdf)