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.