social environments that surround them (e.g., reductions in architectural and attitudinal barriers).

Three other terms and concepts require definition at the outset. Rehabilitation science, as defined in this report for the first time, is the study of movement among states¹ in the enabling-disabling process. This involves the fundamental, basic, and applied aspects of the health sciences, social sciences, and engineering as they relate to (1) the restoration of functional capacity in a person and (2) the interaction of that person with the surrounding environment. Engineering is the application of science and mathematics by which the properties of matter and the sources of energy in nature are made useful to people in machines, products, systems, and processes. (Rehabilitation engineering is a field of engineering that is of fundamental importance to both the restoration of function and the interaction of people with the environment.) Because of the importance of both science and engineering in advancing rehabilitation efforts and addressing the needs of people with disabling conditions, the committee uses the term rehabilitation science and engineering throughout this report to emphasize the importance of both and their synergistic contributions in the process of achieving optimal function.

As originally described by Saad Nagi in the 1950s and refined most recently in the 1991 Institute of Medicine (IOM) report Disability in America, the disabling process has four major components: pathology, impairment, functional limitation, and disability (see Table 1-1). Pathology refers to molecular, cellular, or tissue changes caused by disease, infection, trauma, congenital conditions, or other factors. An example is the death of spinal cord neurons following injury. Impairment occurs at the organ or organ systems level and results in an individual's loss of a mental, physiological, or biochemical function, or abnormalities in these functions. Functional limitation is an inability or hampered ability to perform a specific task, such as climb a flight of stairs.

A disability is defined as a limitation in performing certain roles and tasks that society expects an individual to perform. Disability is the expression of the gap between a person's capabilities and the demands of the environment—the interaction of a person's limitations with social and physical environmental factors. Many disabling conditions are thus preventable or reversible with proper and adequate rehabilitation, including environmental modification. A secondary condition is any additional physical or mental health condition that occurs as a result of having a primary disabling condition. Secondary conditions quite often increase the severity of an individual's disability and are also highly preventable.