Attaining the minimum amount of a given material or energy necessary to achieve a self-sustaining chain reaction under specified conditions.
The term originated in nuclear physics in connection with the amount of fissile material necessary to sustain a fission chain reaction. It may also be used, by analogy, to refer to the quantity of funds, people, information, etc. which must necessarily be assembled before the intensity, variety, and quality of communication in the social subsystem so created is sufficient to maintain mutually stimulating social processes and for the participants to be able to function innovatively and productively – e.g. the critical number of researchers to constitute an innovative research team; of change agents, to sustain a development process; of artists, to constitute a viable artistic community etc.