I feel that the term "rationalism", as opposed to "rationality", or "study of rationality", has undesirable connotations. My concerns are presented well by Eric Drexler in the article For Darwin’s sake, reject "Darwin-ism" (and other pernicious terms):
To call something an “ism” suggests that it is a matter ideology or faith, like Trotskyism or creationism. In the evolution wars, the term “evolutionism” is used to insinuate that the modern understanding of the principles, mechanisms, and pervasive consequences of evolution is no more than the dogma of a sect within science. It creates a false equivalence between a mountain of knowledge and the emptiness called “creationism”.
So, my suggestion is to use "rationality" consistently and to avoid using "rationalism". Via similarity to "scientist" and "physicist", "rationalist" doesn't seem to have the same problem. Discuss.
(Typical usage on Less Wrong is this way already, 3720 Google results for "rationality" and 1210 for "rationalist", against 251 for "rationalism". I've made this post as a reference for when someone uses "rationalism".)
I think people confusing rationalism with one side of the age old rationalism/empiricism debate in philosophy is a much more probable event than someone confusing it with a faith based ideology.
According to Rationality (1989) by Nicholas Rescher, who is for all intents and purposes a rationalist in the sense LW (not academic philosophy) uses the word, the LW rationality is a faith based ideology. See confirmation holism by Quine, outlined in "The Two Dogmas of Empiricism". Rationality is insufficient to justify rationality with rational means, because to do so would presuppose that all means of justification are rational, which already implicitly assumes rationality. Hence, it cannot be refuted that rationality is based on faith. Rescher urges people to accept rationality nevertheless.