The term "evolutionist" is commonly used as an anti-science label by proponents of creationism and intelligent design. Sometimes the word changes to 'evilution' to indicate that belief in evolution is, in some creationist opinions, evil and of the devil. Both "evolutionist" and "evolutionism" refer to scientists and others who accept that the evidence-based theory of evolution is the best explanation for the development of life on the earth (otherwise known as over 99% of all scientists in relevant fields). Often, the term just gets thrown around to refer to anyone else they're disagreeing with at the time, such as atheists or libruls. To compound this stupidity some creationists even argue that "evolutionism" is a secular religion leading to sexual freedom and other supposed failings of present day society.  All in all, this represents mainly a bunch of...
When used in this way it is an attempt to color the argument and imply that evolution is just another belief system or worldview ("ism") as opposed to a scientific theory. This idea of acceptance of evolution being a worldview is quite common, particularly amongst people attempting to have schools "teach the controversy". It is exceptionally typical of the misinformation deliberately generated by proponents of the Wedge Document.
One can only assume that the creationist is saying, in effect: "Your beliefs are just as baseless as mine are, so you might as well believe what I believe." They rarely ever seem to understand that, by their very nature, scientific ideas cannot be beliefs. Science is based on an interpretation of facts, and by definition theories are never beliefs. We may accept evolution, but it would not be science if we believed evolution (or, rather, if one had to believe it regardless of evidence). The distinction is similar to the use of evidence in a court of law: the defendant's mother may believe her son couldn't have committed the crime, but no responsible court will disregard the weight of evidence in favor of someone's unsubstantiated belief about the case.
Furthermore, even if it were a belief system, "evolutionism" would have the minor benefit of being based on physical reality, rather than an old book which claims to be true. Hmmm... one of those just seems to be a bit more rational. Thus, the implication of "equally ridiculous beliefs", even if it weren't basically flawed, still wouldn't hold.
 Self confessed evolutionists
However, the term has also been used by Richard Dawkins, Stephen Jay Gould, PZ Myers and Niles Eldredge as a self-descriptor. AronRa used it in his video "7th Foundational Falsehood of Creationism", though not in reference to himself. In his earlier "5th Foundational Falsehood of Creationism", he criticizes the lumping of all science disagreeable to creationists into "evolutionism."
 See also