1. A weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized
   "hardheads invaded the pasture";
   - lesser knapweed, black knapweed, Centaurea nigra

1. (metallurgy) residual ore that failed to melt during a smelting process
   "The furnace operator had to remove the hardhead before the next batch"
    
2. A steelhead trout, especially in its ocean-going phase
   "Anglers prize hardhead for their fighting spirit"
    
3. An Australian diving duck of the genus Aythya
   "Hardheads are common in wetlands across southern Australia"
    
4. A stubborn or obstinate person
   "He's such a hardhead, he won't listen to reason"
    
5. [Brit] Any of various plants in the genus Centaurea, especially knapweed
   "Hardhead is considered an invasive weed in many areas"

Type of: knapweed

Encyclopedia: Hardheads