A pair of overdots placed over a symbol, as in 
, most commonly used to denote a second derivative with
respect to time, i.e., 
.
Double Dot
See also
OverdotExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Double Dot." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DoubleDot.html