1. A white crystalline double sulphate of aluminium: the potassium double sulphate of aluminium
   "Potassium alum is commonly used in water purification";
   - potassium alum, potash alum
    
2. A white crystalline double sulphate of aluminium and ammonium
   "Ammonia alum is often used as a mordant in dyeing"; "Ammonium alum is commonly used as a mordant in dyeing";
   - ammonia alum, ammonium alum
    
3. A double sulphate of aluminium and potassium that is used as an astringent (among other things)
   "The barber applied alum to stop the bleeding from a small cut"
    
4. A person who has received a degree from a school (high school, college or university)
   "He was a proud alum of the state university";
   - graduate, grad

Derived forms: alums

Type of: Al, aluminium [Brit, Cdn], aluminum [N. Amer], astringent, astringent drug, atomic number 13, double salt, scholar, scholarly person, styptic

Encyclopedia: Alum, Texas