A person or thing holding a pre-eminent position; the best or most important person or thing. Now rare.

The most important part of some business, subject, argument, etc.; the chief matter or principal thing in hand. Cf. main, n.2 1a. Now regional.

Headship, supremacy, sovereignty; chief place, or degree.

A principal object, course, or undertaking in contradistinction to a secondary one (the latter often represented as to bar by and main at by, n.2…

That which is principal; the chief point or part. Obsolete.

With of. The best or choicest part of something.

(In sense 3.) in (also †for, †on, †upon) the main: for the most part; in all essential points; mainly. †the main of all: the most important point…

The chief or principal part of some (material or immaterial) whole; the important or essential point. With of. archaic except in in (also †for, †on,…

The most important feature, or the chief excellence, of a person or thing. Obsolete.

The most important part of or decisive point at issue in a matter, question, etc.; the crux. Usually followed by of. Obsolete.

A staple commodity. transferred and figurative. The thing chiefly ‘dealt in’; the principal object of employment, thought, or discourse…

A point of significance; a crux, a critical turning point; a focal point.

staff and staple: the chief elements or ingredients.

colloquial. Matter of importance or substance; the gist or main part (of a story, situation, etc.). Cf. meaty, adj. 2.

figurative. The chief problem; the central or decisive point of interest.

colloquial. In extended use: the important, essential, or decisive part of an operation, process, period of time, etc.

figurative, esp. the leading part or element (of a thrust, movement, etc.); a person or group leading an attack.