Humidity was the amount of water vapor in the air; where relative humidity was the ratio of the actual amount of humidity present in the air to the maximum humidity possible at the same temperature, expressed as a percentage.
On the planetary file for Draygo IV, it was noted that the planet had a "humidity factor". (TNG: "Homeward")
The ideal moisture level to grow mycelium spores in the cultivation bay was approximately 85% to 87% relative humidity. (DIS: "Despite Yourself")
After Deanna Troi became infected with Barclay's Protomorphosis Syndrome and began transforming into an amphibian, she requested that the ship's computer "increase ambient temperature by two degrees Centigrade and increase relative humidity by ten percent." (TNG: "Genesis")