Change (Δ)
Elementary
Think of a quantity that goes up or down, like the temperature during a day. The change in the quantity is how much it went up or down: its final value minus its starting value.
High school
Consider a quantity $x$ with an initial value and a final value. The change in $x$ is $\Delta x = x_{\text{final}} - x_{\text{initial}}$, the final value minus the initial value. It is negative when the quantity decreases.