WordWeb Online

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

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