  The orbital motion of electrons in a three-dimensional solid can generate a
pseudoscalar magnetoelectric coupling $\theta$, a fact we derive for the
single-particle case using a recent theory of polarization in weakly
inhomogeneous materials. This polarizability $\theta$ is the same parameter
that appears in the "axion electrodynamics" Lagrangian $\Delta{\cal L}_{EM} =
(\theta e^2 / 2 \pi h) {\bf E} \cdot {\bf B}$, which is known to describe the
unusual magnetoelectric properties of the three-dimensional topological
insulator ($\theta=\pi$). We compute $\theta$ for a simple model that accesses
the topological insulator and discuss its connection to the surface Hall
conductivity. The orbital magnetoelectric polarizability can be generalized to
the many-particle wavefunction and defines the 3D topological insulator, like
the IQHE, in terms of a topological ground-state response function.
