Steam is the invisible, hot, gaseous form of water that rises when water boils or evaporates. The word 蒸汽 combines two characters: 蒸 (zhēng), meaning 'to steam' or 'to vaporize', and 汽 (qì), meaning 'vapor' or 'gas'. Together, they specifically refer to water vapor produced by boiling — not other gases or mist. It’s commonly used in scientific, industrial, and everyday contexts, such as describing steam from a kettle, power generation in steam engines, or traditional cooking methods like steaming buns.

Unlike general terms for fog or mist (e.g., 雾 wù), 蒸汽 implies heat-driven phase change from liquid to gas and carries connotations of energy, pressure, or utility. It appears in compound words like 蒸汽机 (steam engine) and 蒸汽锅 (steam cooker). While 汽 alone can mean 'gas' in broader contexts (e.g., 汽油 gasoline), 蒸汽 is precise and unambiguous for 'steam'.