·noun Making hard or harder.
II. Hardening ·p.pr. & ·vb.n. of Harden.
III. Hardening ·noun That which hardens, as a material used for converting the surface of iron into steel.
·add. ·adj Designating, or pert. to, any of various steels that harden when heated to above a red he...
Webster's Dictionary of the English Language