Final answer:
The term describing the temperature at which a liquid starts to vaporize is the boiling point. It is the temperature at which the vapor pressure of the liquid equals the external pressure, and it marks a phase change from liquid to gas.
Step-by-step explanation:
The term used to describe the temperature at which a liquid actively starts to vaporize is C. Boiling point. The boiling point is a critical physical property of a material and refers to the temperature at which the vapor pressure of the liquid equals the external pressure surrounding the liquid, making the liquid change to a gas. This means that when a liquid reaches its boiling point, it will start to produce bubbles of vapor within it, a process often observed when water boils at 100°C (at sea level). Evaporation, on the other hand, can occur at a wide range of temperatures but it does not involve the formation of bubbles throughout the liquid as boiling does. Typically, when a substance reaches its boiling point, a phase change from liquid to gas occurs.