"Naturalism" denotes a number of philsophical stances, typically those descended from materialism and pragmatism, that reject the validity of explanations or theories making use of entities inaccessible to natural science.
As described by W. V. Quine, who is in large measure responsible for naturalism's current success, it is the position that there is no higher tribunal for truth than natural science itself. There is no better method than the scientific method for judging the claims of science, and there is neither any need nor any place for a "first philosophy," such as (abstract) metaphysics or epistemology, that could stand behind and "justify science or the scientific method. Therefore philosophy should feel free to make use of the findings of scientists in its own pursuit; while also feeling free to offer criticism when those claims (Naturalism is not a dogmatic belief that science is entirely correct; it is the position that science is the best explanation we have. We have to start somewhere in talking about the world, and we don't have better evidence for anything other than science; yet as we go along we can still change it as we use it.)
See methodological naturalism and materialism).