Cracking molecules which boil above the threshold temperature into smaller ones which boil below it. Traditionally, the term applied to catalytic crackers. They convert oil which boils above 430 F to hydrocarbons which boil below that point. In other words, they convert gasoil to naphtha. The recent popularity of residue crackers has established another conversion standard around 720 F. This point marks the elevation of fuel oil to light products. Loosely, the term refers to any processing step which breaks molecules into pieces which boil at lower temperatures.