A relational symbol denoting a definition or assignment, combining colon and equals to mean "is defined as."
Examples
Definition of a function or mapping
f(x) \coloneq 2x + 1
Definition of a sequence term
a_n \coloneq \frac{1}{n^2} \text{ for } n \geq 1
Definition in set theory
A \coloneq \{x \in \mathbb{R} : x^2 < 4\}