Bold sans-serif lowercase upsilon

It marks a distinct mathematical symbol, not a fixed value. 𝞄 is Greek lowercase upsilon in bold sans serif. In a formula, it may name a variable; the author's notation says what that variable represents. Its style can distinguish it from plain υ.

Where does Greek upsilon come from?

Its Greek history reaches back to early alphabetic writing. Greek alphabetic writing spread across the Aegean in the 8th century BCE. Upsilon's origin is disputed: a common account derives it from Phoenician waw, a consonant sign repurposed as a Greek vowel; another scholarly view treats upsilon as an independent addition. The Greek letter and the mathematical sans-serif styling are separate stages of its history.

Why can 𝞄 turn into plain υ?

Compatibility normalization can fold the style away. Text processing called compatibility normalization (the NFKC or NFKD Unicode forms) can fold mathematical letter styles back to base Greek or Latin letters. For this character, that may produce υ and merge symbols an equation intended to keep apart. Avoid that conversion when the distinction matters.