Interaction hamiltonian?

Main Question or Discussion Point

A textbook gives the following interaction Hamiltonian describing the interaction of an atom (having transition dipole moment $\mu$) with a photon whose polarization can be $\epsilon_{1}$ or $\epsilon_{2}$):

H = g $\Sigma$$^{2}_{s=1}$$\mu$$\bullet$$\epsilon$s$\sigma^{-}$a$^{+}_{s}$ + h.c.

where $\sigma^{-}$ is Pauli spin-flip operator and a$^{+}_{s}$ is the creation/annihilation operator.

What does "h.c." stand for in this formula?