A collection of sets, considered small or negligible, such that every subset of each member and the union of any two members are also members of the collection.
A subsemigroup with the property that if any semigroup element outside of it is added to any one of its members, the result must lie outside of it.
A subring closed under multiplication by its containing ring.
A non-empty lower set (of a partially ordered set) which is closed under binary suprema (a.k.a. joins).
A perfect standard of beauty, intellect etc., or a standard of excellence to aim at.
A Lie subalgebra (subspace that is closed under the Lie bracket) 𝖍 of a given Lie algebra 𝖌 such that the Lie bracket [𝖌,𝖍] is a subset of 𝖍.
Not actually present, but considered as present when limits at infinity are included.
Optimal; being the best possibility.
Existing only in the mind; conceptual, imaginary.
Pertaining to ideas, or to a given idea.
Perfect, flawless, having no defects.
Teaching or relating to the doctrine of idealism.
The set of members of a fuzzy set that are fully included (i.e., whose grade of membership is 1).
The human clitoris.
The nucleus and electrons of an atom excluding its valence electrons.
For a category with zero morphisms: the equalizer of a given morphism and the zero morphism which is parallel to that given morphism.
A set of pairs of a mapping's domain which are mapped to the same value.
A single seed or grain, especially of corn or wheat.
A small mass around which other matter is concreted; a nucleus; a concretion or hard lump in the flesh.
The core, center, or essence of an object or system.
The central part of many computer operating systems which manages the system's resources and the communication between hardware and software components.
The stone of certain fruits, such as peaches or plums.
A function used to define an integral transform.
The core engine of any complex software system.
The central (usually edible) part of a nut, especially once the hard shell has been removed.
For a given function (especially a linear map between vector spaces), the set of elements in the domain which are mapped to zero; (formally) given f : X → Y, the set {x ∈ X : f(x) = 0}.
To enclose within a kernel
To crenellate