\eqqcolon
The symbol \eqqcolon is used to denote a definition, meaning that the expression on the left-hand side is defined to be the expression on the right-hand side.
Defines 'a' as the sum of 'b' and 'c'.
a \eqqcolon b + c
Defines the function f(x) as a quadratic expression.
f(x) \eqqcolon x^2 + 3x + 2
Defines the set T as the set of all positive real numbers.
T \eqqcolon \{ x \in \mathbb{R} \mid x > 0 \}