The initial boiling point for a distillation fraction is the temperature at which the fraction just begins to evaporate when distilled.

This means it is also in theory the cut point between the fraction and the next lightest fraction being distilled, and is the end point of that lighter fraction.

However, in reality the true initial boiling point of a fraction is typically lower than the true end point of the next lightest fraction (they over lap) because they are complex mixtures of different hydrocarbons. Consequently, refiners in practice split the difference and define an "effective" initial boiling point that is in between and therefore higher than the true initial boiling point.