Step 1: Understand the concept of humidity.
Humidity refers to the amount of water vapor present in the atmospheric air. It is a measure of how much moisture is in the air at a given time and location.
Step 2: Analyze the options.
- (1) Pressure difference in atmospheric air: Humidity is not related to pressure differences. This option is incorrect.
- (2) Amount of water vapour in atmospheric air: This is the correct definition of humidity.
- (3) Temperature in the atmosphere: Humidity is not directly related to temperature. This option is incorrect.
- (4) Quantity of heat energy given by the Sun: Humidity is unrelated to the Sun's heat energy. This option is incorrect.
Final Answer: Humidity refers to the \( \mathbf{\text{amount of water vapour in atmospheric air}} \), which corresponds to option \( \mathbf{(2)} \).