The innermost or central part of a thing; the essential or vital part; the spirit or essence; the core, the nub. Frequently in pith and marrow.

transferred and figurative.

In extended use: the central, vital, or most important part of a thing (now esp. something immaterial).

figurative and in extended use. Something that gives life to or is essential to a person, action, enterprise, etc.; an indispensable factor or…

In singular and (usually) in plural. The essence or core of something. Cf. heart, n. 20. Now rare.

In Aristotle's use: The realization or complete expression of some function; the condition in which a potentiality has become an actuality.

With reference to non-material things: the vital, essential, significant, or operative part; the essence or core (of something).

figurative. Something that gives energy or vitality to a place, institution, enterprise, etc.; a driving force. Also: an essential part or central…

With the. A person who or thing which forms the vital or central part of something; the essence or core. Cf. the life (and soul) of (something, esp…

= heart-blood, n. 2.

= lifeblood, n. 3.