Biopower (or biopouvoir in French) is a term coined by French scholar, philosopher, historian, and social theorist Michel Foucault. It relates to the practice of modern nation states and their regulation of their subjects through "an explosion of numerous and diverse techniques for achieving the subjugations of bodies and the control of populations". Foucault first used the term in his lecture courses at the Collège de France, and the term first appeared in print in The Will to Knowledge, Foucault's first volume of The History of Sexuality. In Foucault's work, it has been used to refer to practices of public health, regulation of heredity, and risk regulation, among many other regulatory mechanisms often linked less directly with literal physical health. It is closely related to a term he uses much less frequently, but which subsequent thinkers have taken up independently, biopolitics, which aligns more closely with the examination of the strategies and mechanisms through which human life processes are managed under regimes of authority over knowledge, power, and the processes of subjectivation.
For Foucault, biopower is a technology of power for managing humans in large groups; the distinctive quality of this political technology is that it allows for the control of entire populations. It refers to the control of human bodies through an anatomo-politics of the human body and biopolitics of the population through societal disciplinary institutions. Initially imposed from outside, whose source remains elusive to further investigation both by the social sciences and the humanities, and in fact, you could argue will remain elusive as long as both disciplines use their current research methods. Modern power, according to Foucault's analysis, becomes encoded into social practices as well as human behavior, as the human subject gradually acquiesces to subtle regulations and expectations of the social order. It is an integral feature and essential to the workings of—and makes possible the emergence of—the modern nation state, capitalism, etc.