1. A factory test designed to catch systems with marginal components before they get out the door; the theory is that burn-in will protect customers by outwaiting the steepest part of the bathtub curve (see infant mortality).
2. A period of indeterminate length in which a person using a computer is so intensely involved in his project that he forgets basic needs such as food, drink, sleep, etc. Warning: Excessive burn-in can lead to burn-out. See hack mode, larval stage.
Historical note: the origin of “burn-in” (sense 1) is apparently the practice of setting a new-model airplane's brakes on fire, then extinguishing the fire, in order to make them hold better. This was done on the first version of the U.S. spy-plane, the U-2.