(or steam injection)
from Greek ἴσος (iso), which means «equal»
→ done at the same temperature
It is a humidification which is done by direct supply of steam from boiling water.
Technically the air does not undergo any temperature variation (if there is no condensation factor of the water vapour).
The sensible energy has been theoretically kept equal, while the latent energy has increased, so the total energy of the air has increased.
In this process the air is passive, the supply of steam is forced (until saturation or even condensation), the air does not lose energy, on the contrary, it gains energy.
Steam is produced by boiling, caused by the direct supply of energy to a volume of water.