Center of a mesh cell
 

I have a structured mesh made of polyhedra (mostly hexahedra, and some pentahedra).
I am trying to find the "center" of each cell of this mesh.
These are irregular polyhedra, where none of the sides/faces are 100% parallel.

In the domain of meshing, when one speaks of the "center" of a cell, what type of center are they referring to? There's many different types of a "center."

For example, there's spatial centers, which is just an average of all the nodes that make up the cell. There's center of volume, center of mass, etc....

You probably mean 'unstructured'. However, the following paper:

http://www.dept.ku.edu/~cfdku/papers/1999-AIAAJ.pdf

has the answer for arbitrary convex polygons/polyhedra.