Mathematical dotless j

It is the dotless italic j used as a separate math symbol.

That describes the glyph's shape, not a fixed quantity. In a formula, it can label a variable—a symbol standing for a value—or another quantity; the surrounding notation explains its role.

Unicode's math chart maps it to TeX's \jmath, a command in a system for preparing math documents.

Why does the j have no dot?

It can distinguish a symbol and settle how an accent looks.

Mathematical typesetting usually removes the dot when an accent mark sits directly over an ordinary j. With a wide hat over a whole expression such as i + j, the writer decides whether j should have a dot; the explicit 𝚥 records the dotless choice. Dotless i and j are also used as independent symbols, without an accent.

A matching dotless j appears in ISO 9573-13, a published list of math-symbol names, and in TeX as \jmath. In 2000, Unicode and STIX, a project supported by scientific and technical publishers, proposed a fuller shared repertoire of math symbols so formulas could move between documents without losing their notation.