What is the "T" set?

Question

I am reading an engineering paper and it references a "T" set in the same way that one would reference the set of complex numbers with $\mathbb{C}$, or the set of real numbers with $\mathbb{R}$". What is this set? I can't find it in any list of set symbols.

Answer 1

$\mathbb{T}$ is sometimes used to denote the circle group, also known as $S^1, U(1)$, or $SO(2)$. This is the group of complex numbers of absolute value $1$ under multiplication.