Actualism is the idea that there is just one possible future, causally closed and determined - even predetermined - by the "fixed past" and the laws of nature.
Actualism denies the existence of alternative possibilities. The opposite of actualism is possibilism.
While the extravagant idea of many possible worlds seems to emphasize the possible, these worlds are all inaccessible from our "actual" world. And in each of the other"possible" worlds, there are no alternative possibilities. Each world is causally closed, pre-determined (possibly) by laws of nature appropriate "at each world."
Philosophers from Diodorus Cronus to Daniel Dennett argue that actualism is "true," so that there is only one possible actual future. The logical truth of a statement now about the future forces the future to keep the statement true. But what if the future makes the statement false?
This is the problem of future contingency, also discussed by Diodorus, but made famous in Aristotle's example of a Sea-Battle in De Interpretatione 9. Aristotle thought statements about the past and present must be either true or false. But statements about the future are only potentials, possibilities, so they lack any truth value until their potential becomes actual at some time in the future.
Note that this means some things in the past can be changed in the future. It is the truth value of a statement that was made in the past. Aristotle's statement "there will be a sea-battle next week," can "actually" be changed if the event does not happen, showing that the concept of a "fixed past," so important in analytic language philosophy debates about free will, has some changeability. In language philosophy, the "fixed past" is far from fixed.
Actualism appeals to philosophers who want the world to be determined by physical laws and by theologians who want the world to be in the hands of an omnipotent, omniscient, and benevolent god. It makes "foreknowledge" of the future possible, or even complete knowledge "outside space and time."
The special theory of relativity, for example, describes a four-dimensional "block universe" in which all the possible events of the future already exist. See, for example, J. J. C. Smart.