\vdots
Vertical dots, often used to indicate continuation in a vertical list or matrix.
Indicating continuation in a matrix column.
\begin{bmatrix} a & b \\ \vdots & \vdots \\ x & y \end{bmatrix}
Indicating continuation in a sequence.
1, 2, \vdots, n
Indicating continuation in a list of functions.
f_1, f_2, \vdots, f_n