I'm pretty sure it's when the terrorists are from the same country they attack. Like Oklahoma City was. In most cases, it also refers to those terrorists attacking targets inside their own country. Thus, bombing a military base owned by country A but inside country B by terrorists originating from country A is usually not known as "domestic terrorism". However, the people performing such an attack are known as "domestic terrorists", as they target things from their own country. It's a subtle difference.