Boil Over is 1) a liquid that is being heated boils over; 2) a violent expulsion of contents caused by a heat wave from the surface burning at the top of the tank reaching the water stratum at the bottom of the tank.
Reference Definition by NFPA: Boil-over is an event in the burning of certain oils in an open-top tank when, after a long period of quiescent burning, there is a sudden increase in fire intensity associated with expulsion of burning oil from the tank.
