That which underlies.

plural of underlier

(phonology) A property of a morpheme: the abstract form that the morpheme is postulated to have before any phonological rules have applied to it.

plural of underlying form

(topology) A set of "points" upon which a topology is built, thereby yielding a topological space. / (algebra) A set upon which some operations are defined, thereby yielding an algebraic structure.

(category theory) a forgetful functor

plural of underlying set

In a way that underlies. / (linguistics) In the underlying form.

The quality of being underlying.