Actualism is the philosophical position that our world is the only real world, and that nothing exists outside of it. Or, to put it in slightly more strict terms, our world is the only world with an existence of the same type as our commonly perceived world.
This is usually a 'bad' idea. I would not Suggest it. Actualism generally boils down to 'this is what I was taught, and I wont change my mind!' Arguments for multiple actually existing universes have support from science, particularly quantum mechanics (for example, the double-slit experiment) and some rather technical philosophical arguments. Actualism is usually based on the same type of bull-headed common sense as the Moorean 'Here is a hand' argument (sometimes this sort of argument is known as a Moorean fact; I hold no truck with this sort of nonsense).
One might, if one were particularly tricky, claim that one is an actualist because one believes that all things exist in multiple actual worlds, one of which is ours. This is an interesting ontological twist; if every universe is made of the same particles and forces, arguably each universe is fundamentally the same universe. This is not what is usually (if ever) ment by actualism, is very very close to being a form of modal realism.
Actualism includes erzatsism and fictionalism.