Darwinism is a term used for various movements or concepts related to ideas of transmutation of species or evolution, including ideas with no connection to the work of Charles Darwin.   The meaning of Darwinism has changed over time, and varies depending on who is using the term.
The term was coined by Thomas Henry Huxley in April 1860, and was used to describe evolutionary concepts, including earlier concepts such as Malthusianism and Spencerism. In the late 19th century it came to mean the concept that natural selection was the sole mechanism of evolution, in contrast to Lamarckism, then around 1900 it was eclipsed by Mendelism until the modern evolutionary synthesis unified Darwin's and Gregor Mendel's ideas. As modern evolutionary theory has developed, the term has been associated at times with specific ideas.
While the term has remained in use amongst scientific authors, it is increasingly regarded as an inappropriate description of modern evolutionary theory    For example, Darwin was unfamiliar with the work of Gregor Mendel , having as a result only a vague and inaccurate understanding of heredity, and knew nothing of genetic drift. In modern usage, particularly in the United States, Darwinism is often used by creationists as a pejorative term.
While the term Darwinism had been used previously to refer to the work of Erasmus Darwin in the late 18th century, the term as understood today was introduced when Charles Darwin's 1859 book On the Origin of Species was reviewed by Thomas Henry Huxley in the April 1860 issue of the Westminster Review. Having hailed the book as, "a veritable Whitworth gun in the armoury of liberalism" promoting scientific naturalism over theology, and praising the usefulness of Darwin's ideas while expressing professional reservations about Darwin's gradualism and doubting if it could be proved that natural selection could form new species, Huxley compared Darwin's achievement to that of Copernicus in explaining planetary motion:
"Darwinism" soon came to stand for an entire range of evolutionary (and often revolutionary) philosophies about both biology and society. One of the more prominent approaches was that summed in the phrase "survival of the fittest" by the philosopher Herbert Spencer, which was later taken to be emblematic of Darwinism even though Spencer's own understanding of evolution was more similar to that of Jean-Baptiste Lamarck than to that of Darwin, and predated the publication of Darwin's theory. What is now called "Social Darwinism" was, in its day, synonymous with "Darwinism" - the application of Darwinian principles of "struggle" to society, usually in support of anti-philanthropic political agendas. Another interpretation, one notably favoured by Darwin's half-cousin Francis Galton, was that Darwinism implied that because natural selection was apparently no longer working on "civilized" people it was possible for "inferior" strains of people (who would normally be filtered out of the gene pool) to overwhelm the "superior" strains, and voluntary corrective measures would be desirable - the foundation of eugenics.
In Darwin's day there was no rigid definition of the term "Darwinism", and it was used by opponents and proponents of Darwin's biological theory alike to mean whatever they wanted it to in a larger context. The ideas had international influence, and Ernst Haeckel developed what was known as Darwinismus in Germany, although, like Spencer Haeckel's "Darwinism" had only a rough resemblance to the theory of Charles Darwin, and was not centered on natural selection at all.
While the reaction against Darwin's ideas is nowadays often thought to have been widespread immediately, in 1886 Alfred Russel Wallace went on a lecture tour across the United States, starting in New York and going via Boston, Washington, Kansas, Iowa and Nebraska to California, lecturing on what he called Darwinism without any problems.
The term Darwinism is often used in the United States by promoters of creationism, notably by leading members of the intelligent design movement  to describe evolution. In this usage, the term has connotations of atheism. For example, in Charles Hodge's book What Is Darwinism?, Hodge answers the question posed in the book's title by concluding: "It is Atheism."   Creationists use the term Darwinism, often pejoratively, to imply that the theory has been held as true only by Darwin and a core group of his followers, whom they cast as dogmatic and inflexible in their belief. Casting evolution as a doctrine or belief bolsters religiously motivated political arguments to mandate equal time for the teaching of creationism in public schools.
However, Darwinism is also used neutrally within the scientific community to distinguish modern evolutionary theories from those first proposed by Darwin, as well as by historians to differentiate it from other evolutionary theories from around the same period. For example, Darwinism may be used to refer to Darwin's proposed mechanism of natural selection, in comparison to more recent mechanisms such as genetic drift and gene flow. It may also refer specifically to the role of Charles Darwin as opposed to others in the history of evolutionary thought - particularly contrasting Darwin's results with those of earlier theories such as Lamarckism or later ones such as the modern synthesis.
In the United Kingdom the term retains its positive sense as a reference to natural selection, and for example Richard Dawkins wrote in his collection of essays A Devil's Chaplain, published in 2003, that as a scientist he is a Darwinist.