Achoo is the sound English speakers write — and sometimes say out loud — when someone sneezes. It is not a word with a meaning so much as a noise spelled out. You use it as an interjection: Achoo! In a comic or a story you can also use it as a noun: a huge achoo. It is informal and a bit playful. British English often writes atishoo instead; it is the same sneeze, different letters. It is not the polite thing you say to someone after they sneeze — that is Bless you, or something like it. Achoo is the sound itself.

哈啾（台灣模擬噴嚏聲；中國大陸較常寫「阿嚏」）