What is the meaning of υ?

Alternative form of ʋ, representing a voiced labiodental approximant.

Lower-case upsilon (ὖ ψιλόν), the 20th letter of the ancient Greek alphabet. It represented the close back rounded vowel /u/ and later the close front rounded vowel /y/. It is preceded by τ and followed by φ.

The lower case letter upsilon (ύψιλον), the 20th letter of the modern Greek alphabet.

Source: wiktionary.org