Sesquipedality is the habit or style of using extremely long, fancy words. Your English teacher might return the rough draft of your paper covered with red marks and a request to tone down the sesquipedality.
You couldn't ask for a longer, fancier, or more complicated word than sesquipedality to describe the use of long, fancy, complicated words. This formal noun comes from the slightly more common adjective sesquipedalian, "long, long-winded, or having many syllables," which was itself originally a noun meaning "something measuring a foot and a half long." The Latin root is sesquipedalia verba, "words a foot and a half long."