1. That which is noxious or just obnoxious fumes or vapor, often associated with putrid swamps or decaying matter, once thought to cause diseases: "In the past, the miasmatically putrid swamps were believed to be the sources of a disease that we now call malaria but which actually means "bad air".
2. A descriptive term for something that is unpleasant, a depressive mood, or an atmosphere resulting from some kind of negative situation.