Question

What is the unit used for water vapor?

Answer 1

A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement.

Complete answer:
Absolute humidity is the measure of water vapor (moisture) in the air. It is expressed as grams of moisture per cubic meter of air ($\dfrac{g}{{{m^3}}}$). The maximum absolute humidity of warm air at ${30^ \circ }C/{86^ \circ }F$ is approximately $30g$of water vapor – $30\dfrac{g}{{{m^3}}}$.
When measuring water vapor, the relative humidity is only a ratio of the amount of water vapor to the maximum amount of water vapor so there isn't really a unit. The absolute humidity can be determined by finding the relative humidity and then looking at the temperature. When you determine this you find out how many grams of water vapor are in the air per kilogram of air.
Hence, the unit used for water vapor is grams/kilogram.

Note:
Obeying Avogadro's Law and the ideal gas law, moist air will have a lower density than dry air. At maximum saturation (that is, relative humidity = $100\%$ at ${0^ \circ }C$) the density will go down to $28.51$g/mol. The water vapor concentration increases significantly as the temperature rises, approaching $100\%$ (steam, pure water vapor) at ${100^ \circ }C$.