Old age refers to ages nearing or surpassing the life expectancy of human beings, and is thus the end of the human life cycle. Terms and euphemisms include old people, the elderly (worldwide usage), OAPs (British usage which stands for Old Age Pensioner), seniors (American usage), senior citizens (American usage), older adults (in the social sciences), and the elders (in many cultures—including the cultures of aboriginal people). Elderly people often have limited regenerative abilities and are more susceptible to disease, syndromes, injuries and sickness than younger adults. The organic process of ageing is called senescence, the medical study of the aging process is called gerontology, and the study of diseases that afflict the elderly is called geriatrics. The elderly also face other social issues around retirement, loneliness, and ageism.