OSI denotes "Open System Interconnect" - A standard put forth by the ISO for communication between computer equipment and networks.
OSI Reference Model - A model put forth by the ISO for communication between computer equipment and networks, which maps out 7 protocol layers.
Top layer: layer number 7: application layer
layer number 6: presentation layer
layer number 5: session layer
layer number 4: transport layer
layer number 3: network layer
layer number 2: data-link layer (e.g. IEEE 802.x)
Bottom layer: layer number 1: physical layer (wire & electricity)
This model explains what each layer does. The model is often used
to explain anyones protocols (not just OSI) to the point where many
people seem to believe that true data-communications requires these
7 layers.
[Source: BIG-LAN Digest, Volume 5, Number 67, Friday, November 5, 1993]