A diagonal pattern of three dots indicating continuation of a pattern along both rows and columns in matrices.
Examples
Diagonal dots in a square matrix
\begin{pmatrix} a_{11} & a_{12} & \cdots & a_{1n} \\ a_{21} & a_{22} & \cdots & a_{2n} \\ \vdots & \vdots & \ddots & \vdots \\ a_{n1} & a_{n2} & \cdots & a_{nn} \end{pmatrix}
Diagonal dots in a triangular matrix
\begin{pmatrix} x_1 & 0 & 0 \\ * & x_2 & 0 \\ * & \ddots & x_3 \end{pmatrix}