A relational symbol denoting a definition or definitional equality, often used in category theory and abstract algebra.
Examples
Definition by equality
f(x) \Doteq x^2 + 2x + 1
Defining a sequence term
a_n \Doteq \frac{n}{n+1} \text{ for } n \geq 1
Matrix equality definition
A \Doteq \begin{pmatrix} 1 & 2 \\ 3 & 4 \end{pmatrix}