Breach of the peace or disturbing the peace, is a legal term used in constitutional law in English-speaking countries and in a public order sense in the several jurisdictions of the United Kingdom. It is a form of disorderly conduct.
In England and Wales, theoretically all criminal offences cognizable by English law involve "a breach of the King's peace", and all indictments formerly concluded "against the peace of our Lord the King, his crown and dignity" before the passage of the Indictments Act 1915 and the Rules that formed that Act's first schedule. The conclusion has also found its way into constitutional law in many United States state constitutions, which mandate that indictments within the state end in a similar manner to the above, usually omitting the "crown" part or substituting "government". For example, New Jersey's is "against the peace of this State, the government and dignity of the same".
Historically that concluding phrase, now legally superfluous, represents the last trace of the process by which the royal courts assume jurisdiction over all offences, and gradually eroded the jurisdiction of the sheriff and of lords of manor and franchises, making crime a matter of national concern as distinguished from civil wrongs or infractions of the rights of local magnates. The Peace of the King was sworn on his accession or full recognition, and the jurisdiction of his courts to punish all violations of that peace was gradually asserted. The completion of this process is marked by the institution of the office of Justice of the Peace.
In England, Wales and Northern Ireland, breach of the peace is descended from the Justices of the Peace Act 1361, which refers to riotous and barratous behaviour that disturbs the peace of the King. More modern authority defines a breach of the peace as "when a person reasonably believes harm will be caused, or is likely to be caused, to a person or in his presence to his property, or a person is in fear of being harmed through an assault, affray, riot, unlawful assembly, or some other form of disturbance".