"Water vapor" refers to the gaseous state of water. It is when water molecules are in the form of gas that cannot be seen with naked eyes. Full definition
They claimed a CO2 increase would cause a temperature increase and higher evaporation with more water vapour in the atmosphere. (drtimball.com)
Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. (skepticalscience.com)
The change in water vapour content will reinforce the warming from orbital change until cloud increases and sets in the negative feedback. (wattsupwiththat.com)