Put simply, natural science (or history) illustration is “art in service of science”. The artist “uses scientifically informed observational, technical, and aesthetic skills to portray a subject accurately.” (The Guild Handbook of Scientific Illustration, by the Guild of Natural Science Illustrators). The scientific illustrator is a “Visual Science Communicator” (Diana Marques). This title aptly describes the key role of a scientific illustrator -- communication! Shapes, anatomy, details, and concepts that cannot be conveyed with words alone, and are often ambiguous in photographs, are visually described by the artist with crystal clarity.
Personally, I have a fondness for the title, "Natural History Illustration", as it puts emphasis on the stories of the visualized subjects, often complex and interwoven.