Is it ensure, insure, or assure?
late 14c., from Old French induracion "hardness, obstinacy" (14c.) or directly from Medieval Latin indurationem (nominative induratio) "hardness (especially of the heart)," noun of action from indurare (see endure).
induration in·du·ra·tion (ĭn'də-rā'shən, -dyə-)
The hardening of a normally soft tissue or organ, especially the skin, because of inflammation, infiltration of a neoplasm, or an accumulation of blood.
A focus or region of abnormally hardened tissue.
hardening of rocks by heat or baking; also the hardening of sediments through cementation or compaction, or both, without the introduction of heat. The classic example is the rock called hornfels, which is formed at contacts with igneous intrusions and in which heat and fluids from the intruding magma reconstitute the original wall rock into a hardened, flinty rock with a dense texture; it also is commonly formed by induration of carbonate sedimentary rocks and shales.