1. A formal set of standards, rules, or formats for exchanging data that assures uniformity between computers and applications. From Glossary of Distance Education and Internet Terminology

2. The rules that govern how things communicate over the network. Key point: By manipulating the protocol raw themselves, hackers can do powerful things that are impossible in an application. For example, client applications typically limit the length of a username that can be typed in. By manipulating the protocol raw, hackers can supply any sized username they want, sometimes causing a buffer overflow exploit. Key point: Protocols are either text-based or binary. Text-based protocols can be read directly off the wire and manipulated directly. Binary protocols require a protocol analyzer to decode them, and must be manipulated programmatically. See also: See the section on "banners" for examples of what some protocols look like on the wire. From Hacking-Lexicon

3. a set of rules that defines exactly how information is to be exchanged between two systems. This allows different types of machine to communicate in a form both understand. From Linux Guide @FirstLinux

4. n. As used by hackers, this never refers to niceties about the proper form for addressing letters to the Papal Nuncio or the order in which one should use the forks in a Russian-style place setting; hackers don't care about such things. It is used instead to describe any set of rules that allow different machines or pieces of software to coordinate with each other without ambiguity. So, for example, it does include niceties about the proper form for addressing packets on a network or the order in which one should use the forks in the Dining Philosophers Problem. It implies that there is some common message format and an accepted set of primitives or commands that all parties involved understand, and that transactions among them follow predictable logical sequences. See also handshaking, do protocol. From Jargon Dictionary