critical.

Inclined to find fault or criticize.

A good teacher is fair but critical.

Pertaining to, or indicating, a crisis or turning point.

This is a critical moment.

Extremely important.

It's critical that you deliver this on time.

Relating to criticism or careful analysis, such as literary or film criticism.

The movie was a critical success, but bombed at the box-office.

Relating to criticism or careful analysis, such as literary or film criticism. Employing or related to textual criticism, particularly through a stemmatological comparison of all extant texts and reconstruction of the original.

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.

The patient's condition is critical.

In such a condition.

The patient is critical.

Two critical after fatal Wimbledon school crash

Likely to go out of control if disturbed, that is, opposite of stable.

The political situation was so critical that the government declared the state of siege.

Of the point (in temperature, reagent concentration etc.) where a nuclear or chemical reaction becomes self-sustaining.

The reaction was about to become critical.

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.

Needing great discrimination to be correctly classified; easily confused.

critical plants

A critical value, factor, etc.

critical hit

In breakdancing, a kind of airflare move in which the dancer hops from one hand to the other.