What is it called when plants lose water through their leaves?

"Transpiration".

Explanation:

Plant loses water by the process of transpiration through the leaves.

The movement of water from different parts of plant through evaporation is called transpiration. this majorly occurs from the stomata.

Transpiration also helps to cool down the heat of the plant during summers, maintains the osmotic pressure and allows the movement of various nutrients through the complete plant body from the upper most shoot to the deep inside roots.

So, the process of transpiration helps the plant to loose water through their leaves.