Here is a list of words that the American Heritage Dictionaries people say every high school graduate should know, obvious link-bait, but I'll bite and I will re-use it for my own nefarious purposes. Of course, it's one of those lists that half the adult you know haven't heard either, even if they aren't particularly obscure.

Among the words:
nomenclature
nonsectarian
notarize
obsequious
oligarchy
omnipotent
orthography
oxidize
parabola
paradigm
parameter
pecuniary
photosynthesis
plagiarize
plasma
polymer
precipitous

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