critical

1. (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.Examples: "The patient's condition is critical."

2. (medicine, by extension) In such a condition.Examples: "The patient is critical."; "Two critical after fatal Wimbledon school crash"broadly

3. (physics) Of the point (in temperature, reagent concentration etc.) where a nuclear or chemical reaction becomes self-sustaining.Examples: "The reaction was about to become critical."

4. (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.

5. (botany) Needing great discrimination to be correctly classified; easily confused.Examples: "critical plants"