From a recent question (Could a programming language be considered as a language?), it came to me the impression that there may be some confusion about the terminology professional linguists use, when referring to the phenomenon of human language.
This is primarily a terminological question. It obviously has conceptual implications, that may be explored here, but my concern is with the use of the expression "natural language" by linguists when it is necessary to differentiate it from other systems of codification invented by humans.
I would be particularly interested in published sources showing the use (or refuse) of this particular term, and/or other references discussing the matter indirectly.
For clarity: there are many different schools of thought in linguistics, that are representative of the field. I'm interested in sources from anyone of them. As you might expect, I do have my own answer, but it is a tentative one, so this question is not just provocative.
Main question: Is the term "natural language", colloquially used to designate human language, informed by linguistic research?
Secondary question: Should the distinction between language in general and specific codification systems be framed by the distinction between nature and artifact?
Philosophers of language and logicians too often dismiss the contributions of linguistics to their field. The same does not happen when the subject is physics. The secondary purpose of this question is to submit to scrutiny the use of the expression "natural language" within the realm of philosophy of language. In that regard, the answer to this question, in this forum, is instrumental.