A disability is any condition that makes it more difficult for a person to do certain activities or interact with the world around them. These conditions, or impairments, may be cognitive, developmental, intellectual, mental, physical, sensory, or a combination of multiple factors. Impairments causing disability may be present from birth or occur during a person's lifetime. The World Health Organization proposes the following definition of disabilities:

"Disabilities is an umbrella term, covering impairments, activity limitations, and participation restrictions. An impairment is a problem in body function or structure; an activity limitation is a difficulty encountered by an individual in executing a task or action; while a participation restriction is a problem experienced by an individual in involvement in life situations. Disability is thus not just a health problem. It is a complex phenomenon, reflecting the interaction between features of a person’s body and features of the society in which he or she lives."
Disability is a contested concept, with different meanings in different communities. The term disability may refer to physical or mental attributes that some institutions, particularly medicine, view as needing to be fixed (the medical model); it may also refer to limitations imposed on people by the constraints of an ableist society (the social model); or the term may serve to refer to the identity of disabled people. Physiological functional capacity (PFC) is a measure of an individual's performance level that gauges one's ability to perform the physical tasks of daily life and the ease with which these tasks are performed. PFC declines with advancing age to result in frailty, cognitive disorders, or physical disorders, all of which may lead to labeling individuals as disabled. According to the World Report on Disability, 15% of the world’s population or 1 billion people are affected by disability.