In mathematics, a net is a generalisation of the concept of a sequence. Recall that a sequence of elements in a set X can be defined as a function on the natural numbers which assigns an element of X to each natural number. A net is a similar object, but with the natural numbers replaced with a more general index set: a net in X is a function from some directed set A into X.

Note that the directed index set A need not be countable or totally ordered, both of which are properties of the natural numbers that render sequences too restrictive for certain mathematical tasks (notably within advanced topology).