Boiling Point is the temperature of a liquid where the gasses embedded in the liquid begin to escape, forming pockets of moving gas. It happens when the vapor pressure of the liquid is equal to the pressure outside of it. The lower the external pressure (i.e. atmosphere), the lower the boiling point. This is different than evaporation that occurs at the contact surface between a liquid and its external environment (i.e. atmosphere) where the gasses escape.