1. The notation of a dot on top of a number or variable typically signifies the derivative with respect to time, a concept used especially in physics and calculus. 2. For example, if $x$ is a function of time $t$, then $\dot{x}$ means $\frac{dx}{dt}$, the rate of change of $x$ over time. 3. This is common in kinematics to represent velocity when $x$ is position. 4. Similarly, $\ddot{x}$ with two dots represents the second derivative with respect to time, often acceleration. Hence, a dot over a number or variable denotes a time derivative.