Lexicographic practices when dealing with minoritized regional languages demonstrably differ in certain key respects from such practices with “big” languages like English or French. Prominent among such differences is the treatment of onomasiological gaps, common in small languages of limited currency, including the handling of neologisms and deliberate coinages. Here we propose a case study of Breton, the regional Celtic language of northwestern France, whose sociolinguistic history has divided intellectuals into two camps with competing visions of authenticity: a traditionalist camp focused on the dialectal speech of older, rural native speakers who have long filled lexical gaps by resorting to borrowing from French; and a modernist camp of younger, urban, well educated “Neo-Breton” speakers, whose standardized Breton eschews French lexical influence, looking rather to the Celtic roots of Breton, and even the closely related language Welsh, to fill lexical gaps. In this case study we examine a selection of quasi-technical terms in a corpus of twentieth- and twenty-first-century bilingual Breton lexicography. The study examines the issue of terminological variation attested for such words, their lexicological profile, and the extent to which their lexicographic handling reflects a position with respect to competing notions of authenticity.