An accent mark placed above a variable to denote a time derivative or rate of change in physics and calculus.
Examples
Time derivative notation (velocity)
\dot{x} = \frac{dx}{dt}
System of differential equations
\begin{cases} \dot{x} = 2x + y \\ \dot{y} = -x + 3y \end{cases}
Newton's second law in mechanics
F = m\dot{v} = m\ddot{x}