A relational symbol denoting a definition or assignment, combining a colon and equals sign to mean "is defined as."
Examples
Definition of a mathematical function
f(x) \coloneqq 2x + 1
Definition of a set
S \coloneqq \{x \in \mathbb{R} : x > 0\}
Definition of a sequence
a_n \coloneqq \frac{1}{n^2} \text{ for } n \in \mathbb{N}