Res extensa (often translated "corporeal substance" by Descartes) is one of the three substances described by René Descartes in his Cartesian ontology, alongside res cogitans and God. Translated from Latin, "res extensa" means "extended thing". In Descartes' substance-attribute-mode ontology, extension is the primary attribute of corporeal substance.
For example, Descartes describes a piece of Wax in the Second meditation. A solid piece of wax has certain sensory qualities. However, when the wax is melted, it loses every single apparent quality it had in its solid form. Still, Descartes recognizes in the melted substance the idea of wax.