Mathematical Script Capital C

It marks a calligraphic C; the formula tells you what it stands for. In math notation, alphabet style can distinguish one symbol from another. The script form does not assign 𝒞 a universal definition; read the surrounding expression to see what that author named with C.

Script alphabets were already used in mathematical and technical print; Unicode did not invent this letter style. In 1999, an encoding proposal argued that separate styled characters would preserve those distinctions in plain-text formulas shared between systems.

Why does script C change shape between fonts?

Its optional variant sequences can specify different script styles. Unicode 18.0 lists two selectors for 𝒞: U+FE00 requests Chancery, while U+FE01 requests Roundhand. Each invisible selector follows the letter; without one, the font supplies its default, so different math fonts can draw different outlines for the same base character.