Grandiloquence is a way of talking that piles on long, formal, important-sounding words — not because the idea needs them, but because the speaker wants to sound big. It is the noun: the habit, or the style, of that kind of speech. Teachers, politicians, and show-offs get accused of it. It is a critical word; almost nobody claims it as a good thing. The pieces are Latin: grandis “great” and a root for “speak” (the same family as eloquent). So it is, almost literally, “great-speaking” — though actually the English meaning is not “speaking well”. Eloquent is praise. Grandiloquence is speaking too grandly.