1. In topology, a subset of a topological space that is a member of the topology; a set where every point has a neighborhood contained entirely within the set
In the real numbers with the standard topology, the interval (0, 1) is an open subset.
Nos números reais com a topologia padrão, o intervalo (0, 1) é um subconjunto aberto.
2. A set that can be expressed as a union of open balls or open intervals in a given metric space
Every open subset of Euclidean space can be written as a union of open balls.
Todo subconjunto aberto do espaço euclidiano pode ser escrito como uma união de bolas abertas.