toughness in WordNet English dictionary

1. impressive difficulty
   formidability  toughness 
   
2. the property of being big and strong
   huskiness  ruggedness  toughness 
   
3. enduring strength and energy
   stamina  staying power  toughness 
   
4. the elasticity and hardness of a metal object; its ability to absorb considerable energy before cracking
   temper  toughness