gen. On which something hinges or depends, fundamental; chief, principal; of special importance, pre-eminent. Chiefly used of abstract things.

Pertinent, relevant; essential. With to, †for.

Most important, chief; fundamental, essential. Cf. primary, adj. 2.

Essential, important. Obsolete.

Of greatest importance; principal or essential. Cf. central, adj. 1.

figurative. That is essential to the existence of something expressed or implied in the context; constituting or involving an essential part or…

Affecting the essence of anything; ‘material’, important.

Central in importance; pivotal, seminal. Cf. nuclear, adj. 2a.

Of paramount or crucial importance; dominant, chief, essential.

Of, relating to, or acting as a pivot; being that on which anything turns or depends; central, crucial, vital.

Originally U.S. Central, vital, most important. Cf. key, adj.

That is the pivot on which something turns or depends; pivotal. Now rare.

Of utmost importance; necessary, essential, indispensable.

That is at the core or heart of something; of, relating to, or constituting the most important or significant part or aspect of something; chief…

attributive passing into adj. That forms (a part of) the core or central area of study, etc.; essential, central, basic, fundamental.