A horizontal ellipsis symbol indicating omitted terms in the middle of a mathematical sequence or expression.
Examples
Series or sequence notation
a_1, a_2, a_3, \cdots, a_n
Matrix elements
\begin{pmatrix} x_{11} & x_{12} & \cdots & x_{1n} \\ x_{21} & x_{22} & \cdots & x_{2n} \end{pmatrix}
Sum with many terms
1 + 2 + 3 + \cdots + n = \frac{n(n+1)}{2}