Nonce borrowing

The term nonce borrowing refers to the use of a word from another language than the main language of the utterance, which has not become an established part of this language. Nonce borrowings are more or less equivalent to instances of single-word codeswitching.

Origin
Apparently the term has been current only since the 1980s. A prominent use occurs in Poplack et al. (1988).

Reference

 * Poplack, Shana & Sankoff, David & Miller, Christopher. 1988. The social correlates and linguistic processes of lexical borrowing and assimilation. Linguistics 26:47-104.

Other languages
German Ad-hoc-Entlehnung