How seldom a thing occurs. Rarity is a fact about frequency that markets mistake for a fact about worth.
Latin raritas (thinness, fewness), from rarus (rare, scattered, thin); rarity was first a fact about how thinly spread something was, before it became, unfairly, a claim about how much it was worth.
How seldom a thing occurs. The root is about spacing, not value, which is exactly what rarity forgets to be whenever it is mistaken for worth.