Why are the computer systems in the OSI model called "open systems"? It might be a silly question, however, why are the computer systems in the X.200 model, OSI, called **open systems**? Note that I've read that **open system** is defined as " _the representation within the reference model of those aspects of a real open system that are pertinent to OSI_ " and the **real open system** is defined as " _A real system which complies with the requirements of OSI standards in its communication with other real systems._ " in the X.200 specification. Thanks in advance.

I would assume that this is because the O in OSI stand for "Open".

Networking is about communication between many different kind of devices.
Using the word "computer" would be misleading, because the OSI model can be applied not only to computers, but to any kind of devices that is able to communicate over the network (routers, manageable swicths, automates, sensors, industrial machines...).

So they had to choose a term to describe _any_ device using the model. The "system" word fit and is vague enough to cover all cases, and so to describe a system compliant to Open System Interconnection, the use "Open system" (which is shorter and more practical that "OSI compliant system")

only a guess tough, I don't think you'll get an authoritative answer from anyone else than the authors.