"The language or rules and conventions that two computers use to pass messages across a network medium. Networking software generally implements multiple levels of protocols layered one on top of another." (Owen Williams, "MCSE TestPrep: SQL Server 6.5 Design and Implementation", 1998)
"A set of rules or standards designed to enable computers to connect with one another and exchange information." (Microsoft Corporation, "SQL Server 7.0 System Administration Training Kit", 1999)
"The way in which two computers transfer data between each other." (Greg Perry, "Sams Teach Yourself Beginning Programming in 24 Hours 2nd Ed.", 2001)
"A list of methods that a class must implement to conform or adopt the protocol. Protocols provide a way to standardize an interface across classes." (Stephen G Kochan, "Programming in Objective-C", 2003)
"A set of rules that govern a transaction." (Marcus Green & Bill Brogden, "Java 2™ Programmer Exam Cram™ 2 (Exam CX-310-035)", 2003)
"A set of semantic and syntactic rules that determines the behavior of functions in achieving communication." (Sharon Allen & Evan Terry, "Beginning Relational Data Modeling" 2nd Ed., 2005)
"A language and a set of rules that allow computers to interact in a well-defined way. Examples are FTP, HTTP, and NNTP." (Craig F Smith & H Peter Alesso, "Thinking on the Web: Berners-Lee, Gödel and Turing", 2008)
"A specification - often a standard - that describes how computers communicate with each other, for example, the TCP/IP suite of communication protocols or the OAI-PMH." (J P Getty Trust, "Introduction to Metadata" 2nd Ed., 2008)
"To communicate effectively, client applications and database servers need a commonly agreed-upon approach. A protocol is a communication standard adhered to by both parties that makes these conversations possible." (Robert D Schneider and Darril Gibson, "Microsoft SQL Server 2008 All-In-One Desk Reference For Dummies", 2008)
"A set of rules that computers use to establish and maintain communication amongst themselves." (Judith Hurwitz et al, "Service Oriented Architecture For Dummies" 2nd Ed., 2009)
"the forms and ceremony used to manage the interaction of elements." (Bruce P Douglass, "Real-Time Agility: The Harmony/ESW Method for Real-Time and Embedded Systems Development", 2009)
"The rules governing the syntax, semantics, and synchronization of communication." (David Lyle & John G Schmidt, "Lean Integration", 2010)
"A list of methods that a class must implement to conform to or adopt the protocol. Protocols provide a way to standardize an interface across classes. See also formal protocol and informal protocol." (Stephen G Kochan, "Programming in Objective-C" 4th Ed., 2011)
"A set of conventions that govern the communications between processes. Protocol specifies the format and content of messages to be exchanged." (DAMA International, "The DAMA Dictionary of Data Management", 2011)
"The standard or set of rules that govern how devices on a network exchange and how they need to function in order to 'talk' to each other." (Linda Volonino & Efraim Turban, "Information Technology for Management" 8th Ed., 2011)
"A standard set of formats and procedures that enable computers to exchange information." (Microsoft, "SQL Server 2012 Glossary", 2012)
"In networking, an agreed-upon way of sending messages back and forth so that neither correspondent will get too confused." (Jon Orwant et al, "Programming Perl" 4th Ed., 2012)
"A set of guidelines defining network traffic formats for the easy communication of data between two hosts." (Mark Rhodes-Ousley, "Information Security: The Complete Reference, Second Edition, 2nd Ed.", 2013)
"A set of instructions, policies, or fully described procedures for accomplishing a service, operation, or task." (Jules H Berman, "Principles of Big Data: Preparing, Sharing, and Analyzing Complex Information", 2013)
"A set of rules controlling the communication and transfer of data between two or more devices or systems in a communication network." (IBM, "Informix Servers 12.1", 2014)
"A rule or custom that governs how something is done. In a computer context, it refers to a standard for transferring data." (Faithe Wempen, "Computing Fundamentals: Introduction to Computers", 2015)
"A set of rules that defines how data is formatted and processed on a network" (Nell Dale & John Lewis, "Computer Science Illuminated" 6th Ed., 2015)
"Defined policies or standards that users adhere to. Protocols are well-defined and accepted procedures. In computer networking, the term refers to algorithms for exchanging various types of data and their interpretation at origination and destination." (Mike Harwood, "Internet Security: How to Defend Against Attackers on the Web" 2nd Ed., 2015)