What does "rare" mean?

Additional senses

1. 2.Of an uncommon nature; unusually excellent; valuable to a degree seldom found. Rare work, all filled with terror and delight. Cowley. Above the rest I judge one beauty rare. Dryden.
2. 3.Thinly scattered; dispersed. Those rare and solitary, three in flocks. Milton.
3. 4.Characterized by wide separation of parts; of loose texture; not thick or dense; thin; as, a rare atmosphere at high elevations. Water is nineteen times lighter, and by consequence nineteen times rarer, than gold. Sir I. Newton. -- Rare, Scarce. We call a thing rare when but few examples, specimens, or instances of it are ever to be met with; as, a rare plant. We speak of a thing as scarce, which, though usually abundant, is for the time being to be had only in diminished quantities; as, a bad harvest makes corn scarce. A perfect union of wit and judgment is one of the rarest things in the world. Burke. When any particular piece of money grew very scarce, it was often recoined by a succeeding emperor. Addison.