In traditional logic, terms are ordered with respect to generality. In decreasing order we have a series such as: thing, body, organism, animal, horse. The infima species is the lowest limit of this process. It may not be obvious why there should be such a limit since the process of subdivision can go on indefinitely. The idea, however, is that after some point further qualifications only add accidents, not essential properties or essences, whereas genuine species are marked off by what is essential.
Philosophy dictionary. Academic. 2011.