Critical

1. Inclined to find fault or criticize
2. Pertaining to, or indicating, a crisis or turning point.
3. Extremely important.

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4. Relating to criticism or careful analysis, such as literary or film criticism.
5. (medicine) Of a patient condition involving unstable vital signs and a prognosis that predicts the condition could worsen; or, a patient condition that requires urgent treatment in an intensive care or critical care medical facility.
6. Likely to go out of control if disturbed, that is, opposite of stable.
7. (physics) Of the point (in temperature, reagent concentration etc.) where a nuclear or chemical reaction becomes self-sustaining.
8. (physics) Of a temperature that is equal to the temperature of the critical point of a substance, i.e. the temperature above which the substance cannot be liquefied.