Why OSI is called Open System?

Open Systems Interconnection (usually abbreviated to OSI) was an effort to standardize networking that was started in 1982 by the International Organization for Standardization (ISO), along with the ITU-T.

It is called Open System because it was intended to be used by all vendors. At that time most network vendors had proprietary protocols that were meant to work only with their software and hardware. This caused many vendors to support multiple types of hardware and software which was complicated, expensive and wasteful.

The OSI standard was meant to improve networking for everybody and was open in that it applied to everyone equally.