Well-being is what is good for a person, or what makes their life go best. It is generally agreed that all plausible moral theories assign at least some value to well-being, with some - welfarist theories - holding that nothing but well-being has value.
There are three popular views of what constitutes a person’s well-being: hedonism, desire-fulfilment theories, and objective list theories.
Hedonism takes well-being to consist of people’s mental states, taking positive experience to be positive well-being and negative experience to be negative well-being. Desire-fulfilment theories (also known as preference satisfaction theories) state that a person’s well-being is constituted by the fulfillment or thwarting of their desires (however that makes them feel). Many desire-fulfilment theories restrict the class of relevant desires to those that are rational and fully informed. Objective list theories hold that a person’s well-being consists in the possession of various objective goods, such as autonomy, friendship, and knowledge.
Measuring well-being is difficult, and the methods for measuring it vary for different theories of well-being. Psychological studies (such as asking people how much enjoyment they are experiencing at particular points in time) may provide insight into this. However, it is more prevalent to use proxy measures such as measures of health. Quality Adjusted Life Years and Disability Adjusted Life Years are (roughly) measures of how healthy a person is, and are frequently used as indicators of welfare.
Moral patienthood considers the related questions of which beings have well-being, and whose well-being people should care about.
Crisp, R. 2013. Well-being. In Edward Zalta (ed.), The Stanford Encyclopedia of Philosophy.
Parfit, Derek. 1984. Reasons and persons. Oxford: Oxford University Press.
Appendix I develops the threefold distinction between hedonism, desire-fulfillment theories and objective list theories.