Vṛddhi (also rendered vr̥ddhi)[1] is a technical term in morphophonology given to the strongest grade in the vowel gradation system of Sanskrit and of Proto-Indo-European.
However, in a later stage of the language this appears to have extended to non-ablauting noun stems that already contained *e, which would contract with the inserted vowel to form a lengthened *ē:[1][4] PIE *swéḱur-o- "father-in-law" (cf.
Latin socer, Sanskrit śváśura) → proto-vṛddhi derivative *swēḱur-ó-s "brother-in-law", lit.
Sanskrit śvāśurá, Old High German swāgur "brother-in-law") The above example also displays the stressing of the thematic vowel when it already exists.
[1] The general phenomenon of vowel gradation, including vṛddhi formation, has been extensively studied and documented as part of Sanskrit's vigorous grammatical tradition, most importantly in the Aṣṭādhyāyī of the grammarian Pāṇini.