DEFINITION of 'Functional Obsolescence'. A reduction in the usefulness or desirability of an object because of an outdated design feature, usually one that cannot be easily changed.The term is commonly used in real estate, but has a wide application.Next Up. reduction in the usefulness or desirability of an object because of an outdated design feature, usually one that cannot be easily changed. The term is commonly used in real estate, but has a wide application. Next Up.