The term used to be previously referred to systems based on UNIX; as contrast to closed systems they included standardized programming interfaces and peripheral interconnects and hence hardware and software development by third party was possible. However today it includes all computer systems that provide interoperability (ability of making systems to work together), portability (adapting software for an environment different from the one for which it was originally designed) and open standards (publicly available standards). Open system refers to platforms that have the following four basic requirements:
1. Must be fully defined so that others can work within the same framework
2. Be stable over a reasonable length of time
3. Must be fully published i.e. interfaces are publicly available
4. Are not under control of any one firm
Open systems can use either open source software or proprietary software. The PC that we use can be an example of open system as although fundamental standards are controlled by Microsoft, Intel and AMD, yet software and hardware applications can be created for the PC. The operating system LINUX is an example of open source (software's source code and rights regarding its redistribution), and the IBM and Hewlett-Packard systems that employ this operating system is an example of open system.