The conventional design procedures aim at finding an acceptable or adequate design that merely satisfies the functional and other requirements of problem. These acceptable designs that satisfy whole the design constraints are called ‘feasible designs’. In general, there will be more than one acceptable design, and the purpose of optimization is to choose the best one of the feasible designs. Thus a criterion has to be chosen for comparing the different alternative feasible designs and for selecting the best one. The criterion, with respect to which the design is optimized, when expressed as a function of design variables, is known as the ‘objective function’.