An LDAP objectClass of type 'ABSTRACT'. From RFC 4512:
An abstract object class, as the name implies, provides a base of characteristics from which other object classes can be defined to inherit from. An entry cannot belong to an abstract object class unless it belongs to a structural or auxiliary class that inherits from that abstract class. Abstract object classes cannot derive from structural or auxiliary object classes. All structural object classes derive (directly or indirectly) from the 'top' abstract object class. Auxiliary object classes do not necessarily derive from 'top'.