TITLE: What does the dot over $x$ or $y$ mean?
QUESTION [14 upvotes]: I am just starting to read up on differential equations. The problem is (in my materials) nowhere is explained what do these dots mean. Can anyone shed some light?
$$\begin{align}&\begin{cases}
\dot x=2x+y ,\\
\dot y=3x+4y. \\
\end{cases} \\
&\begin{cases}
\dot x+x-8y=0, \\
\dot y-x-y=0. \\
\end{cases}\end{align}$$

REPLY [21 votes]: It usually means derivative with respect to time. The notation goes back to Newton. 
By the way, $\ddot{x}$ is often used in Physics for the second derivative of $x$ with respect to time.