PSA Definition of Nature
"Nature Photography is restricted to the use of the photographic process to depict observations from 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 to certify as to its honest presentation. The storytelling value of a photograph MUST be weighed more than the pictorial quality. Human elements shall not be present, except on the rare occasion where those human elements enhance the nature story. The presence of scientific bands on wild animals is acceptable. Photographs of artificially produced hybrid plants or animals, mounted specimens or obviously set arrangements, are ineligible, as is any form of manipulation that alters the truth of the photographic statement.”
PSA Definition of Photojournalism
"Photojournalism images shall consist of pictures or sequences with informative content and emotional impact, including human interest, documentary and spot news. The journalistic value of the photograph shall be considered over pictorial quality. In the interest of credibility, photographs which misrepresent the truth, such as manipulation to alter the subject matter, or situations which are set up for the purpose of photography, are unacceptable in Photojournalism."