An accent mark placing two dots above a variable to denote a second derivative with respect to time in physics and mechanics.
Examples
Second derivative with respect to time (acceleration)
\ddot{x} = -\frac{k}{m}x
Newton's equation of motion
\ddot{r} = -\frac{GM}{r^2}\hat{r}
Simple harmonic oscillator equation
\ddot{y} + \omega^2y = 0