Strength requirements are specified in terms of limit loads (the maximum loads to be expected in service) and ultimate loads (limit loads multiplied by prescribed factors of safety). With respect to aircraft structure and design, ultimate load is the amount of load applied to a component beyond which the component will fail.
The ultimate load is a statistical figure used in calculations, and should (hopefully) never actually occur. The chance that it will occur is, however, not zero, and, if it were to occur, then the aircraft would probably fail.
During the testing for determination of the loads, no fracture is allowed to occur at the ultimate load for a period of 3 seconds.
The ultimate load is related to the limit load Bn, using the concept of safety factor, j. The relation is as follows:
Bu >= 1.5 Bn