A system that allows third parties to make products that plug into or interoperate with it. For example, the PC is an open system. Although the fundamental standards are controlled by Microsoft, Intel and AMD, thousands of hardware devices and software applications are created and sold by other vendors for the PC.
For years, the term "open systems" (plural) referred to the Unix world because Unix ran in more types of computer hardware than any other operating system (combined with Linux, it still does). Contrast with closed system
Open Systems vs. Open Source
Open systems refers to open platforms, whereas open source refers to the software's source code and rights regarding its redistribution. Open systems may employ open source software or proprietary software. See open source
Open Systems vs. Open Standards
Open systems may or may not employ open standards, the Windows PC being the prime example of an open system that is "not" an open standard (governed by a standards organization).
On the other hand, open standards do imply open systems, and the two terms are often used synonymously. However, there is absolutely no reason why an open standard could not be employed within a closed system that cannot be extended or enhanced by a third party. See open standards