Definition:Distance/Points/Real Numbers

Definition
Let $x, y \in \R$ be real numbers.

Let $\left|{x - y}\right|$ be the absolute value of $x - y$.

Then the function $d: \R^2 \to \R$:
 * $d \left({x, y}\right) = \left|{x - y}\right|$

is called the distance between $x$ and $y$.

Also see

 * Distance on Real Numbers is Metric