OSI layer - Definition from the Hotspot Shield Glossary
The OSI or Open Systems Interconnection reference model is used to describe how applications can communicate over a network. It has been implemented so various different platforms and developers can create programs and software that can communicate with each other and interoperate.
A main feature of the OSI Layer model is the 7 distinct layers that data can be split into to allow it to be easily identified and transmitted. These layers are: The Application Layer (to identify communication partners), The Presentation Layer (to describe how the data should be formatted), The Session Layer (to coordinate transmissions), The Transport Layer (to convert the data into packets), The Network layer (to handle the routing), The Data-Link Layer (to establish the network link), and the Physical Layer (provides hardware means of transmitting data).