Ageing (British English) or aging (American English) is the process of becoming older. In the narrow sense, the term refers to biological ageing of human beings, animals and other organisms. In the broader sense, ageing can refer to single cells within an organism (cellular senescence) or to the population of a species (population ageing). In humans, ageing represents the accumulation of changes in a human being over time, encompassing physical, psychological, and social change.