A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchronization of communication and possible error recovery methods.

<networking> The software and/or hardware environment of two or more communications devices or computers in which a particular network protocol operates. A network connection may be thought of as a set of more or less independent protocols, each in a different layer or level. The lowest layer governs direct host-to-host communication between the hardware at different hosts; the highest consists of user application programs. Each layer uses the layer beneath it and provides a service for the layer above. Each networking component hardware or software on one host uses protocols appropriate to its layer to communicate with the corresponding component (its "peer") on another host. Such layered protocols are sometimes known as peer-to-peer protocols. The advantages of layered protocols is that the methods of passing information from one layer to another are specified clearly as part of the protocol suite, and changes within a protocol layer are prevented from affecting the other layers. This greatly simplifies the task of designing and maintaining communication systems. Examples of layered protocols are TCP/IP's five layer protocol stack and the OSI seven layer model. (1997-05-05)

Secure HyperText Transfer Protocol (Reference: HTTP), "Style: S-HTTP"