I haven't seen a formal definition, but I think most would consider landscapes to be largely a subset of nature photography.
The part of landscapes that would not apply would be city scapes or other urban and suburban architectural landscapes, but even these get into a gray area.
Competitions frequently have to define something more specific, so whether or not manmade structures, wildlife, and digital creations are included in landscapes is a matter of choice. I've seen places where HDR and Panos were not considered part of the landscape criteria. I've even seen infrared landscapes excluded. And I don't necessarily agree with these distinctions, but they may represent other categories.