Real Dictionary

Verner's law in American English
an explanation for a series of apparent exceptions to Grimm's law, stating that the Proto-Germanic word-medial voiceless spirants (f, (θ) , h, s), derived from the Proto-Indo-European voiceless stops (p, t, k) and voiceless spirant (s), regularly became v
the statement by K. Verner of a regularity behind some apparent exceptions in the Germanic languages to Grimm's law, namely, that Proto-Germanic voiceless fricatives became voiced when between voiced sounds if the immediately preceding vowel was not accen

Verner's law in British English
noun: a modification of Grimm's Law accommodating some of its exceptions. It states that noninitial voiceless fricatives in Proto-Germanic occurring as a result of Grimm's law became voiced fricatives if the previous syllable had been unstressed in Proto-Indo-E