Criticality is the state of a system where the nuclear chain reaction is precisely self-sustaining, that is, when the reactivity is zero. The term is also used for supercritical states, those in which the reactivity is greater than zero.
In the context of a nuclear reactor, particularly in a nuclear power plant, criticality refers to the normal operating condition of a reactor, in which nuclear fuel sustains a fission chain reaction. A reactor achieves criticality, (and is said to be critical) when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.
The International Atomic Energy Agency also defines the first criticality date as the date when the reactor is made critical for the first time. This is an important milestone in the construction and commissioning of a nuclear power plant.