Generation is the act of producing offspring. In kinship terminology, it is a structural term designating the parent-child relationship. It is also known as biogenesis, reproduction, or procreation in the biological sciences. The term is also often used synonymously with cohort in social science; under this formulation the term means "people within a delineated population who experience the same significant events within a given period of time". Generation in this sense of birth cohort, also known as a "social generation", is widely used in popular culture, and has been the basis for societal analysis. Serious analysis of generations began in the nineteenth century, emerging from an increasing awareness of the possibility of permanent social change and the idea of youthful rebellion against the established social order. Some analysts believe that a generation is one of the fundamental social categories in a society, while others view its importance as being overshadowed by other factors such as class, gender, race, education, and so on.