The boiling point was the temperature at which a substance boiled.
In 2154, Thy'lek Shran recalled visiting planets whose temperature was only slightly below water's boiling point, something Jhamel couldn't imagine. (ENT: "The Aenar")
In 2369, Data performed an experiment to test the aphorism, "a watched pot never boils" by boiling water in a kettle sixty-two times, sometimes watching it, sometimes not. In each case, the water reached its boiling point in 51.7 seconds, thereby disproving the aphorism. (TNG: "Timescape")