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 members of such a category, taken collectively.
A category into which sexually-reproducing organisms are divided on the basis of their reproductive roles in their species.
Genitalia: a penis or vagina.
Another category, especially of humans and especially based on sexuality or gender roles.
The distinction and relation between these categories, especially in humans; gender.
Sexual activity, usually sexual intercourse unless preceded by a modifier.
To determine the sex of (an animal).
To have sex with.
To have sex.