What is the meaning of ν?

coefficient of kinematic viscosity, reluctivity (1/μ), etc.

neutrino

frequency

a dental nasal (IPA [n̪]).

Lower-case nu (νῦ), the 13th letter of the ancient Greek alphabet. It represented the alveolar nasal /n/. It is preceded by μ and followed by ξ.

The lower case letter nu (νι), the 13th letter of the modern Greek alphabet.

Source: wiktionary.org