Core

1 : a central and often foundational part usually distinct from the enveloping part by a difference in nature

the core of the city

: such as
a : the usually inedible central part of some fruits (such as a pineapple); especially : the papery or leathery carpels composing the ripened ovary in a pome fruit (such as an apple)
b : the place in a nuclear reactor where fission (see 1fission 3) occurs
c : an arrangement of a course of studies that combines under basic topics material from subjects conventionally separated and aims to provide a common background for all students

core curriculum

d : the portion of a foundry mold that shapes the interior of a hollow casting
e : a vertical space (as for elevator shafts, stairways, or plumbing apparatus) in a multistory building
f (1) : a computer memory consisting of an array of cores strung on fine wires; broadly : the internal memory of a computer
(2) : a tiny doughnut-shaped piece of magnetic material (such as ferrite) used in computer memories
(3) : a mass of iron serving to concentrate and intensify the magnetic field resulting from a current in a surrounding coil
g : the central part of a celestial body (such as the earth or sun) usually having different physical properties from the surrounding parts
h : a nodule of stone (such as flint or obsidian) from which flakes have been struck for making implements
i : the conducting wire with its insulation in an electric cable
2 a : a basic, essential, or enduring part (as of an individual, a class, or an entity)

the staff had a core of experts

the core of her beliefs

b : the essential meaning : gist

the core of the argument

c : the inmost or most intimate part

honest to the core

3 : a part (such as a thin cylinder of material) removed from the interior of a mass especially to determine composition