Wikipedia entry on Naturalism uses the Oxford English Dictionary definition:-
'Naturalism commonly refers to the philosophical viewpoint that the natural universe and its natural laws and forces (as opposed to supernatural ones) operate in the universe, and that nothing exists beyond the natural universe or, if it does, it does not affect the natural universe that we know.'Metaphysical naturalism (aka ontological or philosophical naturalism) is defined by humanist Paul Kurtz says  :-
1) nature is best accounted for by reference to material principles eg mass, energy, and other physical and chemical properties.
3) there is no "purpose" in nature.Wikipedia describes the distinction between metaphysical & methodological naturalism:-
Metaphysical naturalism holds that there is nothing but natural elements, principles, and relations of the kind studied by the natural sciences, i.e., those required to understand our physical environment by mathematical modeling. Methodological naturalism, on the other hand, refers exclusively to the methodology of science, for which metaphysical naturalism provides only one possible ontological foundation (Ontology is a part of metaphysics asking questions concerning what entities exist or can be said to exist). Metaphysical naturalism holds that all properties related to consciousness and the mind are reducible to nature. The corresponding theological perspective is religious naturalism or spiritual naturalism. Metaphysical naturalism rejects the supernatural concepts and explanations that are part of many religions.Methodological Naturalism says nothing about God's existence according to Paul de Vries of Wheaton college (a conservative evangelical school):-
In 1983 Paul de Vries distinguished between "methodological naturalism," a disciplinary method that says nothing about God's existence, and "metaphysical naturalism," which "denies the existence of a transcendent God."The Panda's Thumb discusses the origins of Methodological Naturalism - the idea may date back to Darwin or even Galileo.
Methodological naturalism is a 'ground rule' of science & the scientific method:-
In 1996, Robert T. Pennock used the term methodological naturalism to clarify that the scientific method confines itself to natural explanations without assuming the existence or non-existence of the supernatural, and is not based on dogmatic metaphysical naturalism as claimed by creationists and proponents of intelligent design. Pennock's testimony at the Kitzmiller v. Dover Area School District trial was cited by the Judge who concluded that "Methodological naturalism is a "ground rule" of science today"Methodological naturalism, the ground rule of science, & nature:-
Methodological naturalism is concerned not with claims about what exists (= metaphysical naturalism) but with methods of learning what is nature. It is strictly the idea that all scientific endeavors—all hypotheses and events—are to be explained and tested by reference to natural causes and events. The genesis of nature, e.g., by an act of God, is not addressed. Methodological naturalism (cf. metaphysical naturalism) seeks only to provide a framework within which to conduct the scientific study of the laws of nature. Methodological naturalism is a way of acquiring knowledge. "since the scientific revolution of the 16th and 17th centuries, science has been limited to the search for natural causes to explain natural phenomena.... While supernatural explanations may be important and have merit, they are not part of science." Methodological naturalism is thus "a self-imposed convention of science." It is a "ground rule" that "requires scientists to seek explanations in the world around us based upon what we can observe, test, replicate, and verify."