1. Scholastic usage (logic). A class of things contained in a genus possessing some attribute called the “specific difference” that distinguishes it from other species contained in the genus. E.g., “rational animal” is a species contained in the genus “animal.” Its specific difference is “rational.”  As is also the case for genus, according to classical logic, species is a relation of reason (relation rationis) formed by the intellect, correlative to the relation of genericity.  That is, the genericity of animal in relation to man implies the specificity of man in relation to animal. 

2. Scholastic usage (cognition). The thing known insofar as it is present to and, hence, in some manner “in,” the cognitive power as determining the power to know the thing. 

Intelligible Species (also called Species).— The species proper to the intellect. It is constituted by the essence of the thing known and is the formal cause of intellectual knowledge and is abstracted by the agent intellect (which is the proximate efficient cause of intellectual knowledge) from the phantasm (which is the objective-instrumental cause of intellectual knowledge). It is a universal. 

Sensible Species (also called Species).— The species proper to the external senses or internal senses. It is the formal cause of sensible knowledge. 

Impressed Species (in Latin, Species Impressa).— The species proper to the senses or the intellect as present in them and capable of determining them to know the thing of which it is the species. 

Expressed Species (in Latin, Species Expressa).— The species proper to the internal senses or to the intellect as present therein and determining them actually to know the thing of which it is the species. In the intellect, it is equivalent to the formal concept.  The expressed intellectual species is also referred to as the verbum mentis, verbum cordis, conceptus mentis.  

3. Modern biological usage. The lowest level classification of living things. It is the class below genus.