Latin:res = thing, cogitan = to think, res cogitans = thinking thing. This phrase is used in philosophy when discussing mental substance. The concept held by dualists and idealists is that minds are made-up of non-physical substance (i.e., consciousness). This is opposed to the materialists, who believe that what we normally think of as mental substance is ultimately physical matter (i.e., brains).
Descartes proposed that consciousness resides within an immaterial domain he called res cogitans (the realm of thought), in contrast to the domain of material things which he called res extensa (the realm of extension).