as a liquid is heated its vapour pressure increases to the point at with it just equals the applied pressure (usually atmospheric pressure). at this point the liquid is observed to boil.
the normal boiling point is measured at 760 mmhg (760 torr) or 1 atm. at a lower applied pressure the vapour pressure needed for boiling is also lowered and the liquid boils at a lower temperature. the relation between applied pressure and temperature of boiling for a liquid is determined by its vapour pressure-temperature behaviour as shown in figure