\begin{definition}[Definition:Error/Absolute]
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:
{{begin-eqn}}
{{eqn | n = 1
      | l = \varepsilon
      | o = :=
      | r = X - x
}}
{{eqn | n = 2
      | l = \varepsilon
      | o = :=
      | r = x - X
}}
{{eqn | n = 3
      | l = \varepsilon
      | o = :=
      | r = \size {X - x}
}}
{{end-eqn}}
where $\size {X - x}$ denotes the absolute value of $X - x$.
Different sources use different conventions.
\end{definition}