Definition: A device used to measure the relative humidity in the air. There are several different types of hygrometers available, includings ones marketed specifically for use in terrariums.
Hygrometers are quite delicate and need to be calibrated for accurate measuring. They are also not instant devices - it can take up to two hours to get an accurate reading or to record changes in the relative humidity.
More: See How to Calibrate a Hygrometer
What is Relative Humidity?