Abstract and
concrete are classifications that denote whether a term describes an object with a physical
referent or one with no physical referents. They are most commonly used in
philosophy and
semantics. Abstract objects are sometimes called
abstracta (sing.
abstractum) and
concrete objects are sometimes called (sing.
concretum). An
abstract object is an
object which does not exist at any particular time or place, but rather exists as a
type of thing,
i.e., an
idea, or
abstraction. The term 'abstract object' is said to have been coined by
Willard Van Orman Quine. The study of abstract objects is called
abstract object theory.