WordWeb Online

1. Extraordinarily good or great; used especially as an intensifier
   "a marvelous collection of rare books"
   - fantastic, grand, howling, marvellous [Brit, Cdn], rattling, terrific, tremendous, wonderful, wondrous
    
2. Too improbable to admit of belief
   - improbable, marvellous [Brit, Cdn], tall
    
3. Being or having the character of a miracle
   - marvellous [Brit, Cdn], miraculous

See also: extraordinary, incredible, marvel, supernatural, unbelievable

Encyclopedia: Marvelous