Natural history is the scientific research of plants or animals, grouped among the natural sciences. It consists of the study of any category of natural objects or organisms. Today’s field is more of a cross discipline umbrella of many specialty sciences such as systematics (zoology, botany etc), evolution, biogeography or sedimentology, thus implying a strong multi-disciplinary approach.
The way I perceive Natural History today, is a combination of geology and biology (which are sciences that require the understanding of the other fundamental sciences: mathematics, physics, chemistry). If a naturalists gets in a certain spot on earth, he will be able to understand and define this particular environment.