rarity

Etymology

rare + -ity, borrowed from Middle French rarité, from Latin rāritās; compare French rareté. See also rare.

noun

1. A measure of the scarcity of an object.
2. (chemistry, of a gas) Thinness; the property of having low density
   Only the increasing rarity bothered him, and he thought that perhaps it was this which had turned the heads of other travellers and excited those absurd tales of night-gaunts whereby they explained the loss of such climbers as fell from these perilous paths. 1927, H. P. Lovecraft, The Dream-Quest of Unknown Kadath
3. A rare object.
   Plant breeding is always a numbers game.[…]In addition, we are looking for rare alleles, so the more plants we try, the better. These rarities may be new mutations, or they can be existing ones that are neutral—or are even selected against—in a wild population. A good example is mutations that disrupt seed dispersal, leaving the seeds on the heads long after they are ripe. 2013 May-June, David Van Tassel, Lee DeHaan, “Wild Plants to the Rescue”, in American Scientist, volume 101, number 3