A class of information systems considered in this paper is defined as follows: a system belongs to the class if its change can be caused by both its environment and factors inside the system, and there is an information transfer from it to its environment and from its environment to it. Two formalisms (information transition systems and conceptual transition systems) for abstract unified modelling of the artifacts (concept sketches and models) of the conceptual design of information systems of the class, early phase of information systems design process, are proposed. Information transition defines the abstract unified information model for the artifacts, based on such general concepts as state, information query, answer and transition. Conceptual transition systems are a formalism for conceptual modelling of information transition systems. They defines the abstract unified conceptual model for the artifacts. The basic definitions of the theory of conceptual transition systems are given. A language of conceptual transition systems is defined.