阿嚏 is the onomatopoeia for a sneeze in Chinese, equivalent to 'achoo!' or 'atishoo!' in English. It's typically used as an interjection — you say it when someone sneezes in conversation, or you can write it in quotation marks in narrative text to describe the sound. It's often followed by a polite response like '是谁在想你？' or '有人念叨你' in Chinese culture.