1.not widely known; especially valued for its uncommonness;
a rare word
2.recurring only at long intervals;
a rare appearance
3.not widely distributed;
rare herbs
4.marked by an uncommon quality; especially superlative or extreme of its kind;
what is so rare as a day in June-J.R.Lowell
5.having low density;
rare gasses
6.(of meat) cooked a short time; still red inside;
rare roast beef