Criticality is the state of a nuclear fission chain reaction where the number of neutrons produced in each generation is sufficient to maintain the reaction rate. This condition is quantified by the neutron multiplication factor, which must equal one for a self-sustaining process. As a fundamental concept in nuclear physics and reactor engineering, criticality describes the balance between neutron production and loss, determining whether a system is subcritical (reaction dying out), critical (steady-state reaction), or supercritical (reaction increasing), and is thus crucial for reactor control, safety, and achieving controlled energy release.