WordWeb Online

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

See also: extraordinary, incredible, supernatural, unbelievable

Encyclopedia: Marvellous