A computer-based system with the defining characteristic that it provides information to users in one or more organizations. Information systems are thus distinguished from, for example, real-time control systems, message-switching systems, software engineering environments, or personal computing systems.

The term could have a very much wider meaning than that suggested, considering the range of meaning of the words information and system. It could, for instance, be broadened to include all computer-based systems, or further broadened to include many non-computer-based systems. Thus, within the domain of computer-based systems, the more specific term organizational information system is sometimes used.

Information systems include data processing applications, office automation applications, and many expert system applications. When their primary purpose is to supply information to management, they are commonly called management information systems.

The following are among the more important characteristics of information systems, and make their design and construction particularly difficult.

1. (a) Their environment is complex, not fully definable, and not easily modelled.

2. (b) They have a complex interface with their environment, comprising multiple inputs and outputs.

3. (c) The functional relationships between inputs and outputs are structurally, if not algorithmically, complex.

4. (d) They usually include large and complex databases (or, in future, knowledge bases).

5. (e) Their ‘host’ organizations are usually highly dependent on their continuing availability over very long periods, often with great urgency attending their initial provision or subsequent modification.