Protocol (network): a communication standard for network/host devices that facilitates messaging between entities.
It is difficult to generalize about protocols because they vary so greatly in purpose and sophistication. Most protocols specify one or more of the following properties:
Wikipedia Definition of Network Protocol
Some Common Protocols examples (***Not an exhaustive list):
Network Layer 3 Protocols:
Layer 2 LAN Protocols:
Layer 2 protocols are responsible for converting data packets passed to it by the Network/Internet Layer (above) into binary data in frames. This data is passed to the Physical Layer.
A routing protocol is a set of rules or standard that determines how routers on a network communicate and exchange information with each other, enabling them to select best routes to a remote network.
And MANY MANY more not listed.