  The semi-annihilation reaction takes the schematic form psi_i psi_j -> psi_k
phi, where psi_i are stable dark matter particles and phi is an unstable state.
Such reactions are allowed when dark matter is stabilized by a larger symmetry
than just Z_2. They lead to non-trivial dark matter dynamics in the early
universe, and the thermal production of the relic particles can be completely
controlled by semi-annihilations. This process might also take place today in
the Milky Way, enriching the (semi-)annihilation final state spectrum observed
in indirect detection experiments.
