Initial boiling point
Also known as: IBP
The initial boiling point for a distillation fraction is the temperature at which the fraction just begins to evaporate when distilled.
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 overlap) 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.