Three different numbers that all describe "how humid" — and only one of them stays honest when the temperature changes.
Relative humidity is the number everyone knows, and it's also the number that quietly lies to you. It isn't a direct measurement of how much water is in the air at all — it's a ratio that moves whenever temperature moves, even if not one molecule of water vapor is added or removed. Dew point and wet-bulb temperature are the two numbers engineers reach for when they actually need to know what's going on with the moisture itself, and each one answers a slightly different question. Mixing the three up is one of the most common sources of bad calls in psychrometrics — from dehumidification sizing to why a "low RH" reading can still mean a very wet day.