I have a friend studying a language from the pacific islands, and she found an affix that when added to a noun makes a verb and when added to a verb makes a noun. What would you call such a thing, and how might one find it in the literature?
I would suggest calling this/these affix(es) either a nominalising affix or a verbalising affix, depending on its context. (See Additional Information below.)
These terms are already present in literature as seen below:
Morphological Productivity (Cambridge; Google Books) gives examples of nominalisation affixes in English:
-ment, -tion, -ity
... and examples of "verbifying/verbalising" affixes:
(see link for contents; gives lots of linguistic examples)
Languages do map different morphological processes to sometimes the same phone. For example in English, we have -s, which is called either a plural(-ising) affix or a possessive affix, depending on its context, because it will either turn a singular word plural or it will signify possession of a noun. Therefore, approaching this problem as a native-English speaker, I stick true to the aforementioned suggestions.
However, jlawler mentions above:
It's not always clear what's a noun and what's a verb in Austronesian languages.
With this new information in mind, perhaps a native speaker of an Austronesian language does not necessarily differentiate nouns/verbs as I do. This difference might then promote your friend to call this paradigm something specific to Austronesian languages. If such a term does not exist in specific literature, then typically your friend can simply coin an appropriate term.
Thanks for the answers and comments! My friend is going with the term derivational affix (abbreviation DER) as used by Karen Ashley (who works on the Sa'a language of the Solomon Islands). Seems a bit vague, but I guess it was necessary to show it is the same morpheme performing both functions. I'll try to release more info when the paper is published. :-)