Definition
Latin: "not twice for the same." A foundational maxim of civil law origin holding that no person may be tried twice for the same offense. Once a competent tribunal has reached a final judgment — whether conviction or acquittal — the matter is closed and cannot be relitigated against the same defendant.
Non bis in idem is the civil law and international law expression of a principle that common law systems recognize through the doctrine of double jeopardy and the related pleas of autrefois convict and autrefois acquit. Though the phrase originates in Roman law, it operates today as a general principle of criminal procedure across civilian jurisdictions, international criminal tribunals, and transnational legal instruments.
Common Confusion
Non bis in idem and double jeopardy express the same underlying protection but are not interchangeable in research. Double jeopardy is the operative constitutional term in American law; non bis in idem is the phrase used in civil law systems, international criminal law (including the ICC Statute and the ICCPR), and comparative legal scholarship. Researchers who encounter non bis in idem in a common law context are almost certainly reading a source engaging with international, civilian, or comparative material — not a domestic American criminal procedure source. Conversely, common law sources discussing double jeopardy rarely use the Latin phrase. The two terms track different literatures and should be searched separately.
A secondary confusion: non bis in idem is sometimes loosely equated with res judicata. Both protect against relitigation of resolved matters, but res judicata operates primarily in civil proceedings as a preclusion doctrine. Non bis in idem is specific to criminal prosecution and carries the additional protection that even a procedurally incomplete prior proceeding may bar retrial in some civilian systems — a result res judicata analysis would not necessarily reach.
Why It Matters in Research
The phrase is a reliable marker of civilian or international legal context. In historical common law sources — including English cases and early American materials — you will find the underlying concept discussed through double jeopardy, autrefois convict, and autrefois acquit rather than through this Latin phrase. If the term appears in a common law historical source, read carefully: it may signal borrowing from civilian doctrine, scholarly commentary, or a comparative passage.
In modern research, non bis in idem is central to international criminal law. The Rome Statute (Article 20) codifies the principle for ICC proceedings. The International Covenant on Civil and Political Rights (Article 14(7)) embeds it as a human rights protection. Researchers working on extradition, international tribunal jurisdiction, or transnational criminal enforcement will encounter the principle as a potential bar to proceedings where a defendant has already been tried in a foreign jurisdiction — a question that double jeopardy doctrine under U.S. constitutional law largely does not reach (given the dual sovereignty doctrine).
The scope of what counts as "the same offense" varies considerably across systems and is a live analytical problem in both comparative scholarship and international criminal proceedings. Researchers should not assume that the identity-of-offense analysis used in American double jeopardy cases maps cleanly onto non bis in idem analysis in civilian or international sources.
Historical Dictionary Support
The four historical dictionaries converge on the core meaning without significant divergence: no person may be tried twice for the same offense, and the protection applies once a tribunal of last resort has rendered a final judgment of conviction or acquittal.
Bouvier's is the most substantive. He situates the phrase explicitly in civil law, cites the Code (9.2.9.11), references Merlin's Répertoire, and cross-references JEOPARDY — signaling that he understood the civilian maxim and the common law doctrine as parallel expressions of the same principle. His requirement of a "tribunal in the last resort" is notable: it suggests finality is an element of the protection and anticipates questions about what proceedings are sufficient to trigger the bar.
Black's (2nd ed.) makes the connection to common law double jeopardy explicit, calling non bis in idem an expression of "the same principle" as double jeopardy. This is useful framing for researchers: Black's treats the two as equivalent in substance even while flagging the different vocabulary.
What the historical dictionaries do not address: the international dimension, the scope of "same offense" under comparative analysis, and the application of the principle to proceedings before supranational bodies. These are twentieth- and twenty-first-century developments beyond the reach of the shelf sources.
Jurisdictional Note
In American constitutional law, the functional equivalent is the Double Jeopardy Clause of the Fifth Amendment, and non bis in idem as such has no operative domestic role. In European Union law, the Court of Justice has developed a substantial body of doctrine applying non bis in idem across member state jurisdictions. International criminal tribunals apply the principle under their own statutory frameworks, which may or may not align with domestic implementations.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Double Jeopardy; International Criminal Law; Res Judicata