The space 
which is the subset of 
that is the union of the simplices in a simplicial
complex 
.
The term polytope is sometimes used as a synonym for
underlying space (Munkres 1991, p. 8).
Underlying Space
See also
Polyhedron, PolytopeExplore with Wolfram|Alpha
References
Munkres, J. R. Analysis on Manifolds. Reading, MA: Addison-Wesley, 1991.Referenced on Wolfram|Alpha
Underlying SpaceCite this as:
Weisstein, Eric W. "Underlying Space." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/UnderlyingSpace.html