Humidity
Humidity is the amount of water vapor in the air, often expressed as relative humidity: the percent of the maximum the air can hold.
Definition
Humidity measures how much water vapor, invisible water in gas form, is in the air, and high humidity makes air feel sticky because sweat cannot evaporate off the skin easily. Relative humidity is the ratio of the water vapor present to the maximum the air can hold at that temperature, expressed as a percent, and because warm air holds far more vapor than cold air, relative humidity rises simply as air cools toward its dew point, the temperature at which it becomes saturated. Saturation vapor pressure itself rises nearly exponentially with temperature under the Clausius-Clapeyron relation, about $7\%$ per degree Celsius, and condensation in the atmosphere depends on latent heat release of about $2{,}260\ \text{kJ/kg}$, which is what powers thunderstorms and hurricanes.
Example
On a humid summer day in Charlotte, sweat lingers on the skin and the air feels hotter, while in dry Arizona air the same sweat evaporates fast and feels cool, and dew forms on grass overnight as the ground cools nearby air down to its dew point. Dew points above $70^\circ F$ are common in a Charlotte July, and a hurricane draws its energy from evaporating warm ocean water, with condensation in its eyewall releasing latent heat at a rate comparable to hundreds of times the world's electrical generating capacity.
Key Insight
Fog and clouds are not water vapor itself but tiny liquid droplets formed once vapor cools and condenses, and relative humidity can shift dramatically overnight with no water added at all, simply because the air cooled. The Clausius-Clapeyron relation also explains why a warmer climate brings heavier downpours: each degree of warming lets the atmosphere hold roughly $7\%$ more moisture, and when that extra moisture is wrung out, the rain falls harder.