Monism

Monism is the philosophical view that only one sort of stuff in the universe exists ultimately. Monism is to be distinguished from dualism, which holds that ultimately there are two kinds of stuff, and from pluralism, which holds that ultimately there are many kinds of stuff.

Monism is often seen as partitioned into three different kinds:

Physicalism or materialism, which holds that only the physical is real, and that the mental can be reduced to the physical
Idealism or phenomenalism, which holds the converse
Neutral monism, which holds that both the mental and the physical can be reduced to some sort of third, more "neutral" kind of stuff

Certain other positions are hard to pigeonhole into the above categories, including:

Functionalism (philosophy of mind), which like materialism holds that the mental can ultimately be reduced to the physical, but which holds that all the critical aspects of mind as also reducible to some substrate-neutral "functional" level. Thus something need not be made out of neurons to have mental states. This is a popular stance in cognitive science and artificial intelligence.
Eliminativism, which holds that talk of the mental will eventually be proved as unscientific and completely discarded. Just as we no longer follow the ancient Greeks in saying that all matter is composed of earth, air, water, and fire, people of the future will no longer speak of "beliefs", "desires", and other mental states. A subcategory of eliminativism is radical behaviourism, a view held by B. F. Skinner.)
Anomalous Monism, a position proposed by Donald Davidson in the 1970s as a way to resolve the Mind-body problem. It could be considered (by the above definitions) either physicalism or neutral monism. Davidson hold that here is only physical matter, but that all mental objects and events are perfectly real and are identical with (some) physical matter. But physicalism retains a certain priority, inasmuch as (1) All mental things are physical, but not all physical things are mental, and (2) (As John Haugeland puts it) Once you take away all the atoms, there's nothing left. This monism was widely considered an advance over previous identity theories of mind and body, because it does not entail that one must be able to provide an actual method for redescribing any particular kind of mental entity in purely physical terms. Indeed there may be no such method; this is a case of nonreductive physicalism.

For some, monism may also have religious/spiritual implications. For example, it can be argued that pantheism is essentially a monistic view. Recognizing this, some inveigh against the 'dangers of monism,' asserting that in order to resolve all things to a single substrate, one dissolves God in the process.

Historically, monism has been promoted in spiritual terms on several occasions, most notably by Ernst Haeckel. To the dismay of most modern observers (contemporary monistic thinkers in particular), Haeckel added various proto-Nazi ideas to his presentation of monism, thereby assuring the ultimate demise of his Monistic Alliance.

There is a growing undercurrent of monism in the modern spiritual and philosophical climate, evidenced by increasing Western fascination with Taoism, Buddhism, Pantheism, Zen, and similar systems of thought which explore the mystical and/or spiritual elements of a monistic philosophy.