Plax (πλάξ) means a flat surface, tablet, or slab. In the New Testament it is used exclusively to refer to tablets — specifically the stone tablets of the Law given to Moses and the "fleshy tablets" of the human heart upon which God writes His new covenant. The word evokes the image of a writing surface, whether stone or flesh.
Paul's use of plax in 2 Corinthians 3:3 creates one of the most powerful contrasts in the New Testament: the old covenant was written on tablets of stone (plaxin lithinais), but the new covenant is written on tablets of human hearts (plaxin kardias sarkinais). This echoes the prophetic promise of Jeremiah 31:33 and Ezekiel 36:26 — God would replace hearts of stone with hearts of flesh and write His law internally. The shift from stone plax to heart plax represents the entire transition from old covenant to new, from external law to internal transformation by the Spirit. In Hebrews 9:4, the word refers to the stone tablets inside the Ark of the Covenant, connecting back to the Sinai revelation.