For meanings in specific fields, see protocol (computing) or protocol (cryptography).

In addition to the meaning below, treaties are sometimes called protocols. The word derives from a Greek phrase meaning "first leaf", referring to the first draft of a treaty.

Protocol is the etiquette of diplomacy and affairs of state. A protocol is a rule which guides how an activity should be performed, especially in the field of diplomacy. In the diplomatic and government fields of endeavor protocols are often unwritten guidelines. Protocols specify the proper and generally-accepted behavior in matters of state and diplomacy, such as showing appropriate respect to a head of state, ranking diplomats in chronological order of their accreditation at court, and so on.

In international law and international relations, a protocol is a treaty or international agreement that supplements a previous treaty or international agreement. A protocol can amend the previous treaty, or add additional provisions. Parties to the earlier agreement are not required to adopt the protocol; sometimes this is made clearer by calling it an 'optional protocol', especially where many parties to the first agreement do not support the protocol. Some examples: the United Nations Framework Convention on Climate Change (UNFCCC) established a framework for the development of binding greenhouse gas emission limits, while the Kyoto Protocol contained the limits later agreed upon.

By the turn of the twenty-first century the word protocol had also come into wide use in the computer and communications fields; see communications protocol and protocol (computing).