An abstract term or concept; an abstraction; (occasionally) spec. an abstract noun. Frequently opposed to concrete.

Something considered or expressed in abstract terms; something which exists only as an idea or in theory; an abstract concept or idea; (also…

Without article. Abstract knowledge or principles, as opposed to practical experience or activity; theorizing, theoretical speculation.

An abstract entity or property; esp. an abstract term; = abstract, n. 1.

A non-thing, vague abstraction, or concept having no properties.