Definition:Error/Absolute

Definition
Let $x$ be an approximation to a (true) value $X$.

The absolute error $\varepsilon$ is the difference between $x$ and a $X$, and can be defined in one of three ways:

where $\size {X - x}$ denotes the absolute value of $X - x$.

Different sources use different conventions.

Also see

 * Definition:Relative Error