core (nom)
Sens 1 : the central and often foundational part of a body, mass, or construction
usually distinct from the enveloping part by a difference in nature or by being cut out or
separated. [source : MWU]
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Sens 2 : the part (as of an individual, a class, an entity) that is basic,
essential, vital, or enduring as distinct from the incidental or transient. [source : MWU]