Final answer:
The boiling point is the temperature at which a liquid starts to vaporize, occurring when the liquid's vapor pressure equals the external pressure.
Step-by-step explanation:
The term used to describe the temperature at which a liquid actively starts to vaporize is called the boiling point. This is characterized as the temperature at which the vapor pressure of a liquid equals the external pressure above the liquid. As a liquid heats up, the particles gain kinetic energy, increasing the rate of evaporation. When the entire liquid reaches enough energy to overcome intermolecular forces, vaporization occurs. This process is key in the phase change from liquid to gas and involves the enthalpy of vaporization or heat of vaporization (symbolized as ^Hvap), which is the energy required for the phase transition.