Chiefly Law. Of evidence or a fact: significant or influential, esp. in having affected a person's decision-making; (U.S. Law) having a logical…

Of a thing: main, principal, chief. Now rare.

Of a point or period of time: of the highest or critical importance. Now rare.

Constituting the ‘stamina’ or main outlines of a subject.

Chiefly U.S. Of an issue, topic, etc.: key, fundamental; likely to rouse strong feelings.