A description logic concept, which corresponds to a class in OWL or a formula with one free variable in FOL, is either an atomic concept or a non-atomic concept. The definition of non-atomic concepts varies between description logics. In ALC, for example, the set of concepts includes all atomic concepts (every *A* ∈ N_{C}), the top concept (⊤), and the bottom concept (⊥). If *C* and *D* are concepts and *R* ∈ N_{R}, then the intersection of two concepts (*C* ⊓ *D*), the union of two concepts (*C* ⊔ *D*), the complement of concepts (¬*C*), the universal restriction of concepts by a role (∀*R*.*C*), and the existential restriction of concepts by a role (∃*R*.*C*) are also concepts.