The Greek letter $\Delta$ is used in mathematics to express

**change** of a variable.

If $x$ changes from $x$ to $x'$ we write $\Delta x=x'-x$. Notice that when $x$ increases $\Delta x>0$, and when $x$ decreases $\Delta x< 0$.

Example

If $x$ passes from 2 to 4 it has increased by 2 units: $\Delta x =2$.

If $x$ passes from 4 to 2 it has decreased by 2 units: $\Delta x =-2$.