Noun
(grammar, linguistics, phonology) Euphonic nu /ν/ (n). Consonant /ν/ affixed to vowel endings of some Ancient Greek grammatical forms to produce euphony: in prevocalic position, to avoid sounding two vowels in a row (hiatus), and prevent elision or fusion of vowels contracting into one sound (crasis). In preconsonantal coda position, /ν/ may be added to create a long syllable in poetic meter. Nu is also usually written before a pause at the end of a clause in prose or the end of a verse in poetry.
(grammar, linguistic morphology) Paragogic nu /ν/ (n), understood as an allomorph of certain Greek morphemic forms otherwise terminating in vowels /ε/ (e) or /ι/ (i), viz. third person verb and third declension dative plural noun forms, phonemically represented as -ε(ν) (-e(n)) and -ῐ(ν) (-i(n)). An early Attic–Ionic innovation, nu paragoge spread through the Hellenic world; inclusion or omission of /ν/ in ancient inscriptions is so variable and unpredictable as to suggest morphemes with and without /ν/ occurred in free variation, not bound to the prescriptive rules of phonetic harmony codified by later Byzantine grammarians.
Source: en.wiktionary.org