Let $X$ be a set. A net is a map from a directed set to $X$. In other words, it is a pair $(A,\gamma)$ where $A$ is a directed set and $\gamma$ is a map from $A$ to $X$. If $a\in A$ then $\gamma(a)$ is normally written $x_{a}$, and then the net is written $(x_{a})_{{a\in A}}$, or simply $(x_{a})$ if the direct set $A$ is understood.

Now suppose $X$ is a topological space, $A$ is a directed set, and $(x_{a})_{{a\in A}}$ is a net. Let $x\in X$. Then $(x_{a})$ is said to converge to $x$ if whenever $U$ is an open neighbourhood of $x$, there is some $b\in A$ such that $x_{a}\in U$ whenever $a\geq b$.

Similarly, $x$ is said to be an accumulation point (or cluster point) of $(x_{a})$ if whenever $U$ is an open neighbourhood of $x$ and $b\in A$ there is $a\in A$ such that $a\geq b$ and $x_{a}\in U$.

Nets are sometimes called Moore–Smith sequences, in which case convergence of nets may be called Moore–Smith convergence.

If $B$ is another directed set, and $\delta\colon B\rightarrow A$ is an increasing map such that $\delta(B)$ is cofinal in $A$, then the pair $(B,\gamma\circ\delta)$ is said to be a subnet of $(A,\gamma)$. Alternatively, a subnet of a net $(x_{\alpha})_{{\alpha\in A}}$ is sometimes defined to be a net $(x_{{\alpha_{\beta}}})_{{\beta\in B}}$ such that for each $\alpha_{0}\in A$ there exists a $\beta_{0}\in B$ such that $\alpha_{\beta}\geq\alpha_{0}$ for all $\beta\geq\beta_{0}$.

Nets are a generalisation of sequences, and in many respects they work better in arbitrary topological spaces than sequences do. For example:

• If $X$ is Hausdorff then any net in $X$ converges to at most one point.

• If $Y$ is a subspace of $X$ then $x\in\overline{Y}$ if and only if there is a net in $Y$ converging to $x$.

• if $X^{{\prime}}$ is another topological space and $f\colon X\rightarrow X^{{\prime}}$ is a map, then $f$ is continuous at $x$ if and only if whenever $(x_{a})$ is a net converging to $x$, $(f(x_{a}))$ is a net converging to $f(x)$.

• $X$ is compact if and only if every net has a convergent subnet.