Rare

There are two main uses of the word rare. The first refers to something that is 'scarce', 'uncommon' or 'not very close together'. This can be applied to an animal - for example, the red panda could be said to be rare in that there are not many in existence. This word can also be applied to particles and more commonly refers to gases - for example when climbing a mountain the air becomes rarefied in that the density of the air is decreasing, containing fewer molecules. In the study of chemistry, a rare earth is "any naturally occurring oxide of an element of the lanthanide series (usu[ally] including lanthanum and freq[uently] also scandium and yttrium); also (loosely), any of these elements themselves; a lanthanide. Hence rare-earth element, metal" (OED).

The other use of the word rare usually applies to meat and means underdone or undercooked - for example, someone may ask for their steak to be cooked "rare" and what they mean is that they want the middle of the meat to remain quite uncooked, and pink in colour. The British English word for this used to be underdone, rare in this use being largely confined to American English; but this seems to be changing.