The core, center, or essence of an object or system.

(botany) The central (usually edible) part of a nut, especially once the hard shell has been removed.

(botany) A single seed or grain, especially of corn or wheat.

(botany, US) The stone of certain fruits, such as peaches or plums.

A small mass around which other matter is concreted; a nucleus; a concretion or hard lump in the flesh.

(computing) The central part of many computer operating systems which manages the system's resources and the communication between hardware and software components.

(computing) The core engine of any complex software system.

(programming) The simplified input to an algorithm that has undergone kernelization.

(calculus) A function used to define an integral transform.

(mathematics) A set of pairs of a mapping's domain which are mapped to the same value.

(mathematics, linear algebra, functional analysis) For a given function (especially a linear map between vector spaces), the set of elements in the domain which are mapped to zero; given f : X → Y, the set {x ∈ X : f(x) = 0}.

(mathematics, category theory) For a category with zero morphisms: the equalizer of a given morphism and the zero morphism which is parallel to that given morphism.

(mathematics, fuzzy set theory) The set of members of a fuzzy set that are fully included (i.e., whose grade of membership is 1).

(slang) The human clitoris.

(chemistry) The nucleus and electrons of an atom excluding its valence electrons.

To enclose within a kernel

To crenellate