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 ∈ NC), the top concept (⊤), and the bottom concept (⊥). If C and D are concepts and R ∈ NR, then the intersection of two concepts (CD), the union of two concepts (CD), 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.