In western art, negative space is the area within the work that does not significantly reflect the subject matter. In a figure/background work, the background is considered to be less important. The figure is more important and therefore is considered to be the positive space. In some cases, the subject matter of the work is the entire work and there is no negative space. This is often the case in landscapes such as this work by Thomas Cole.
Bear in mind that all cultures do not view positive and negative space in the same way.