Mathematical sans-serif bold small t

It is a bold sans-serif t used as its own math letter.

A bold t can denote a vector (a quantity with direction and size), a matrix (a rectangular array of values), or something else, depending on the writer’s convention. The formula’s definitions tell you what this t denotes.

Why is its style encoded as a character?

In formulas, type style can be part of the symbol.

In 1873, James Clerk Maxwell proposed German Fraktur (blackletter) to distinguish vectors—quantities with direction and size—from scalars, which have size alone. Oliver Heaviside used bold type in 1886 as a clearer alternative. A 1999 Unicode proposal by Murray Sargent and Barbara Beeton argued for encoding whole mathematical alphabets so plain text could preserve style distinctions that change a formula’s meaning. This 𝘁 belongs to that mathematical alphabet.