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1. Extraordinarily large in size or extent or amount or power or degree
   "tremendous sweeping plains"; "a tremendous fact in human experience; that a whole civilization should be dependent on technology"; "a plane took off with a tremendous noise"
   - enormous, ginormous [Brit]
    
2. Extraordinarily good or great; used especially as an intensifier
   "a tremendous achievement"
   - fantastic, grand, howling, marvelous [US], marvellous [Brit, Cdn], rattling, terrific, wonderful, wondrous
    
3. Extreme in degree or extent or amount or impact
   - frightful, terrible, awful

See also: big, extraordinary, large, tremendously

Encyclopedia: Tremendous