Depending on what sort of “space” is, this may be defined in different ways.
If is a topological space in the classical sense, then it is defined by specifying a collection of subsets to be called “open” (that are closed under finite intersections and arbitrary joins, i.e. are a sub-frame of the power set of ).
If is a convergence space (or some variation such as a subsequential space), we define a subset to be open if any filter/net/sequence converging to a point of must be eventually in . This defines an “underlying topological space” of .
The idea is that this subspace is the part of which involves only , and we may identify with when we are looking only at . If is a (sober) topological space regarded as a locale, then any such open sublocale is spatial and coincides with the subspace determined by the subset (and this is true even in constructive mathematics).
See open subtopos.
One is to use a dominance, which specifies the open subsets representably by a set of “open truth values”.
Another possibility is the following definition due to Penon: is open if for any and , either or . This definition does not require a choice of dominance, but it is generally only correct for Hausdorff spaces; for instance, the open point in the Sierpinski space is not open in this sense. However, in various gros toposes of topological, smooth, or algebraic spaces it does induce the correct notion of open subsets for Hausdorff spaces; see Dubuc-Penon.
Jacques Penon, Topologie et intuitionnisme