A nuclear unit becoming critical means when a chain reaction that causes automatic splitting of the fuel nuclei has been established.

or

A nuclear reactor is said to be critical when the neutron population in the reactor core is constant.

The reactor core must be designed so that the mass of fuel will be just sufficient to ensure one neutron from each fission causing fission in another atom. A mass and configuration of fissionable material in which this occurs are termed the ‘critical mass’, or reactor in which this condition is achieved is said to have ‘gone critical’.

Criticality is the normal operating condition of a nuclear reactor, in which nuclear fuel sustains a fission chain reaction. A reactor achieves criticality (and is said to be critical) when each fission releases a sufficient number of neutrons to sustain an ongoing series of nuclear reactions.