Open set
From Conservapedia
An open set, in the kind space we're used to, is a set such that, intuitively, if a point is contained in an open set, then all points sufficiently close to it are also contained.
Formally, we say that a set is an open set if it contains a ball around each of its points.[1]
Open sets are the basic objects in topology, in terms of which all other objects are defined.
