Generally, someone who claims that various sorts of things that are not realised completely in our (sensory) experience are real. The things in question might be, e.g.: numbers, infinite constructions, material objects, theoretical entities (atoms, the unconscious mind, etc. ) and so on. During the middle ages "realist" specifically meant someone who maintained that there are universals who maintained that there are universals (e.g."horsiness" "humanity") corresponding to words such as "horse" and "human" and not just individual things. See nominalist.