A naturalistic artlang is an artlang that is designed to resemble a natlang. A good naturalistic artlang has a grammar as complex as grammars of natlangs and avoids unnatural features such as self-segregating morphology or a taxonomic vocabulary. Famous examples of naturalistic artlangs are Quenya, Sindarin, Brithenig and Verdurian. Many naturalistic artlangs are diachronic conlangs and come with a fictional history and an elaborated conculture associated with it; some conlangers create entire families of interrelated naturalistic artlangs.
This usage of the adjective 'naturalistic' is to be distinguished from the usage among auxlangers.
The Naturalist Manifesto
Jesse Bangs posted a manifesto to the CONLANG mailing list on March 11, 2002:
I whole-heartedly subscribe to the first of the three goals stated therein, naturalness. The second goal, completeness, warrants some criticism. As in most other arts, there is merit in both large (complete conlangs) and small forms (sketchlangs) of conlanging. I'd always prefer an ingenious and original sketchlang over a language with 10,000 words that turns out to be a relex of English (or Lojban, or Esperanto). The third goal, creativity, is of course self-evident, but I don't think it can be measured by typological distance from the languages one is familiar with. Of course, relexes and euroclones done out of laziness are bad. But that doesn't mean one has to go on an anti-L1 binge and cram every single 'exotic' feature one can find into one's conlang. Take Germanech, for example. That language does nothing not found in various western European languages. Everything you can find in Germanech occurs in German, in French or in some other major European language. But that only makes sense - it is meant to be a pretty ordinary European language, because that's where it is set, and it is intended as a Romance language that has undergone the characteristic sound changes of German. And those changes won't turn Vulgar Latin into a bizarre monster with clicks, split-ergative morphosyntax, or whatever. Everything else than a "humdrum European language" would not be what is called for here.