A vector space $E$, generally over the field $\mathbb R$ or $\mathbb C$ with a map $\lVert \cdot\rVert\colon E\to \mathbb R_+$ satisfying some conditions.

A vector space $E$, generally over the field $\mathbb R$ or $\mathbb C$ with a map $\lVert \cdot\rVert\colon E\to\mathbb R_+$ is a normed space if the three conditions are fulfilled:

- $\lVert x\rVert =0\Rightarrow x=0$,
- For all $x\in E$ and for all $\lambda\in\mathbb R$ (or $\mathbb C$), $\lVert\lambda x\rVert=|\lambda |\lVert x\rVert$,
- For all $x,y\in E$, $\lVert x+y\rVert\leq\lVert x\rVert+\lVert y\rVert$.

For instance, in $\mathbb{R}^n$ each of the following functions is a norm:

- $\displaystyle\bigl\|(x_1,x_2,\ldots,x_n)\bigr\|_2=\sqrt{{x_1}^2+{x_2}^2+\cdots+{x_n}^2}$;
- $\displaystyle\bigl\|(x_1,x_2,\ldots,x_n)\bigr\|_1=|x_1|+|x_2|+\cdots+|x_n|$;
- $\displaystyle\bigl\|(x_1,x_2,\ldots,x_n)\bigr\|_\infty=\max${$|x_1|,|x_2|,\ldots,|x_n|$}.