|A 1985 definition of monism, as contrasted with dualism, according to George Scott. |
The specific term "monism" is a product of the early 17th century, a reaction to Cartesian-initiated dualism (c.1610) arguments about how humans are split into two parts: body + mind / soul divide.
In this sense, historically, albeit not in specific namesake, Parmenides and Benedict Spinoza are said to be representative “monism” in theoretical logic. (Ѻ) Greek philosopher Thales’ c.570BC water principle philosophy, i.e. his theory “all is water” seems to be one example of a monism conceptualized philosophy.
Wolff | Etymology
The term “monism” was coined by German mathematical philosopher Christian Wolff (1679-1754), supposedly in his 1728 Logic (Ѻ), and or his 1734 paper “Psycholgica Rationalis” (Ѻ), supposedly, following Gottfried Leibniz and his earlier "monad" or monadology theories. 
German physicist Ludwig Buchner’s view that force and mind emanated from original matter has been referred to as “materialistic monism” (or monistic materialism): 
“In Buchner’s statement: ‘just as a steam engine produces motion, so the intricate organic complex of force-bearing substance in an animal organism produces a total sum of certain effects, which, when bound together in a unity, are called by us mind, soul, thought’, he postulates force and mind as emanating from original matter—a materialistic monism. But in other parts of his works he suggests that mind and matter are two different aspects of that which is the basis of all things—a monism which is not necessarily materialistic, and which, in the absence of further explanation, constitutes a confession of failure. Büchner was much less concerned to establish a scientific metaphysic than to protest against the romantic idealism of his predecessors and the theological interpretations of the universe. Nature according to him is purely physical; it has no purpose, no will, no laws imposed by extraneous authority, no supernatural ethical sanction.”
In the context of social physics, monism, according to Pitirim Sorokin (1928), is the idea that theories of physics explain both human and non-human worlds, i.e. "psychical and social phenomena are mere variations of natural phenomena". 
In 1887, German physicist Georg Helm published his The Doctrine of Energy, wherein he devotes a section to monism.
Haeckel | Ostwald
In 1906, Ernst Haeckel founded the “Monisten Bund” or German Monist League (Ѻ), in Jena, a free-thinking organization centered on promoting some type of monistic world view, anchored in some way on a mix of Goethe’s 1809 metamorphology theory, Darwin's 1959 theory of evolution, and Wilhelm Ostwald's 1898 energetics view. (Ѻ)
In 1910, Haeckel elected Ostwald as the president of the Monist League, after which he began giving his famous “Monistic Sunday sermons”, as Haeckel (1913) called them. Ostwald, in this direction, was interested in educational reforms and in monism. He believed that in view of his position he could decisively fight the Church's claim to power in the field of natural sciences and to spread a modern scientific ideology. This aim he pursued in his writings Monistische Sonntagspredigten (Monistic Sunday sermons) and Arbeiten zum Monismus (Works on Monism). (Ѻ)
German-cultures scholar Carl Krockel argues that English novelist and literary critic D.H. Lawrence (1885-1930) was “certainly aware of Goethe’s theory of affinities, if only through reading of Ernst Haeckel’s account of it in The Riddle of the Universe” and argues that Haeckel’s pantheism helped Lawrence break from his Christian upbringing, for a monism based on the material universe. The Lukacs work mentioned above, to note, seems to be his 1968 Goethe and His Age. 
The term “theistic monism” (or religious monism) is view that God or some other type of religious concept is behind the one underlying principle. Spinoza would seem to be an example of this view.
1. Merriam-Webster Collegiate Dictionary, 2000.
2. (a) Sorokin, Pitirim. (1928). Contemporary Sociological Theories (§1: The Mechanistic School (pdf), pgs. 4-62; monism, pg. 3). Harper & Brothers.
(b) Barnes, Trevor J. and Wilson, Matthew W. (2014). “Big Data, Social Physics, and Spatial Analysis: the Early Years” (pdf) (abs) (pdf), in: Big Data & Society, Jun.
3. (a) Buchner, Ludwig. (1855). Force and Matter, 7th edition (pg. 130). Publisher.
(b) Buchner, Friedrich Karl Christian Ludwig – Encyclopedia Britannica (1911).
4. Wolff, Christian. (1728). Logic: Rational Thoughts on the Powers of the Human Understanding: with Their Use and Application in the Knowledge and Search for Truth (monism). George Olms Verlag, 2003.
5. Krockel, Carl. (2007). D.H. Lawrence and Germany: the Politics of Influence (pgs. 21-22). Rodopi.
6. Scott, George P. (1985). Atoms of the Living Flame: an Odyssey into Ethics and the Physical Chemistry of Free Will (monism, pg. 29). University Press of America.
● Monism – Wikipedia.