Medical Definition of critical
1a: relating to, indicating, or being the stage of a disease at which an abrupt change for better or worse may be anticipated with reasonable certainty <the critical phase of a fever>b: being or relating to an illness or condition involving danger of death <critical care> <a critical head injury>
2a: of sufficient size to sustain a chain reaction—used of a mass of fissionable materialb: sustaining a chain reaction—used of a nuclear reactor
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