Vacuity is the noun for being vacuous — emptiness of thought. If a speech has nothing real inside it, if a person says only pretty nothing-words, if a TV show has no idea in it at all, you can call that vacuity. It is a strong, cold, formal word, and it is an insult. A writer will speak of "the vacuity of modern political talk" or "the sheer vacuity of the argument". You are not saying the thing is wrong; you are saying there is nothing there to be right or wrong about. Usually uncountable: the vacuity of his remarks. You will meet it mostly in reviews, essays, and serious criticism. Almost nobody uses it in speech; in conversation people say "it was empty", "there was nothing in it", "total fluff". Though actually — it also has an older, plainer meaning of "an empty space", and doctors and scientists once used it that way. Today that sense is rare, and "vacuum" or "empty space" has taken over. If you meet vacuity in a modern text, it is almost certainly about mind, not about space.

（言論、思想）空洞、言之無物；空虛。用來批評內容沒有實質、講了等於沒講。