15,842 new words

tarn : A mountain lake. But specifically a lake that is created by an amphitheater-like hole dug by a glacier, and then filled in with rain water. The word comes from Old Norse, tjorn, meaning pond. So the English took the Norse word for pond and used it to rename a pond.
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Read in So You Want To Be A Wizard: 

Strange creatures like phoenixes and psammeads, moving under smoky London daylight of a hundred years before, in company with groups of bemused children; starships and new worlds and the limitless vistas of interstellar night, outer space challenged but never conquered; princesses in silver and golden dresses, princes and heroes carrying swords like sharpened lines of light, monsters rising out of weedy tarns, wild creatures that talked and tricked one another.  .  .  .

psammead was another new word in that passage; it's a sand-fairy that apparently existed only in the 1902 book Five Children And It, a sort of pre-Narnian book about five kids in England who meet a magical creature and have all kinds of mishaps and adventures. Apparently Five Children And It has been continuously in print for 113 years. I'd never heard of it before, but as soon as I read about it I borrowed it from the library, so I'll let you know how it is.