rare in WordNet English dictionary

1. marked by an uncommon quality; especially superlative or extreme of its kind
   "what is so rare as a day in June"-J.R.Lowell; "a rare skill"; "an uncommon sense of humor"; "she was kind to an uncommon degree"
   rare  uncommon 
   
2. recurring only at long intervals
   "a rare appearance"; "total eclipses are rare events"
   rare 
   
3. (of meat) cooked a short time; still red inside
   "rare roast beef"
   rare 
   
4. not widely distributed
   "rare herbs"; "rare patches of green in the desert"
   rare 
   
5. having low density
   "rare gasses"; "lightheaded from the rarefied mountain air"
   rare  rarefied  rarified 
   
6. not widely known; especially valued for its uncommonness
   "a rare word"; "rare books"
   rare