The process by which a plant loses water: water travels up from the roots, through the stem, and escapes as vapour through tiny holes on the surface of the leaves. It is basically a plant sweating and breathing out water at the same time — and it is not waste. That upward pull of escaping water is what drags more water and minerals up from the soil, which is how a tall tree gets water to its top leaves with no pump. This is a biology word, and for most students it arrives in a science class, often in the same lesson as photosynthesis and respiration. The three sit together and are easy to mix up: photosynthesis makes food using light, respiration releases energy from food, transpiration moves and loses water. Related phrases you will meet: 'transpiration rate', 'the transpiration stream', and in geography 'evapotranspiration' (water lost from soil and plants together). Outside science the word is rare. There is an older, more literary use meaning something becoming known or an event happening — but that belongs to the verb 'transpire', and if you say 'transpiration' in normal conversation, people will assume you are talking about plants.

（植物的）蒸散作用 — 水從根部上升、由葉片氣孔以水蒸氣散失。生物課用語，和光合作用、呼吸作用常一起考。