A relational symbol denoting a definition where the left side is being defined equal to the right side.
Examples
Defining a mathematical function
f(x) \Coloneqq 2x + 1
Defining a set by its properties
S \Coloneqq \{x \in \mathbb{R} : x^2 < 4\}
Defining a sequence
a_n \Coloneqq \frac{1}{n^2} \text{ for } n \in \mathbb{N}