The definition of nature is that defined by the Photographic Alliance of Great Britain (PAGB) in February 2018 [http://www.thepagb.org.uk/wp-content/uploads/nature_definition.pdf]:
Note that images already meeting the separate FIAP Definition of Nature will satisfy this less restrictive definition.
Nature photography is restricted to the use of the photographic process to depict all branches of natural history, except anthropology and archaeology, in such a fashion that a well-informed person will be able to identify the subject material and certify its honest presentation.
The story telling value of a photograph must be weighed more than the pictorial quality while maintaining high technical quality.
Scientific bands, scientific tags or radio collars on wild animals are permissible. Photographs of human created hybrid plants, cultivated plants, feral animals, domestic animals, or mounted specimens are ineligible, as is any form of manipulation that alters the truth of the photographic statement.
Processing of the captured image, by cropping, exposure adjustment, colour correction, noise minimisation, dodging/burning, HDR, focus stacking and sharpening, is allowed. Cloning of image defects and minor distractions, including overlapping elements, are permitted when these do not distort the truth of the photographic statement.
Images entered as Nature can have landscape, geologic formations, weather phenomena, and extant organisms as the primary subject matter. This includes images taken with the subjects in controlled conditions, such as zoos, game farms, botanical gardens, aquariums and any enclosure where the subjects are totally dependent on man for food.
Access to biological subjects may be restricted. By entering a PAGB event, Photographers warrant that they have followed relevant codes of practices and hold any necessary licences.
Guidance on implementation and interpretation of the PAGB Definition of Nature is available separately, and may be read in conjunction with this Definition.
Images entered in Wildlife sections are further defined as one or more extant zoological or botanical organisms free and unrestrained in a natural or adopted habitat. Landscapes, geologic formations, photographs of zoo or game farm animals, or of any extant zoological or botanical species taken under controlled conditions are not eligible in Wildlife sections. Wildlife is not limited to animals, birds and insects. Marine subjects and botanical subjects (including fungi and algae) taken in the wild are suitable wildlife subjects, as are carcasses of extant species. Wildlife images may be entered in Nature sections.