harden in WordNet English dictionary

1. become hard or harder
   "The wax hardened"
   harden  indurate 
   
2. make hard or harder
   "The cold hardened the butter"
   harden  indurate 
   
3. make fit
   "This trip will season even the hardiest traveller"
   season  harden 
   
4. cause to accept or become hardened to; habituate
   "He was inured to the cold"
   inure  harden  indurate 
   
5. harden by reheating and cooling in oil
   "temper steel"
   temper  harden