Definitions of rare:

• adjective:   marked by an uncommon quality; especially superlative or extreme of its kind

  Example: "What is so rare as a day in June-J.R.Lowell"

• adjective:   having low density

  Example: "Rare gasses"

• adjective:   not widely distributed

  Example: "Rare herbs"

• adjective:   not widely known; especially valued for its uncommonness

  Example: "A rare word"

• adjective:   (of meat) cooked a short time; still red inside

  Example: "Rare roast beef"

• adjective:   recurring only at long intervals

  Example: "A rare appearance"