Definition
Latin: "from the beginning." A legal characterization applied to acts, agreements, estates, or status that are treated as having been valid or invalid from their very inception — not merely from the point at which they are declared to be so. When a court or rule of law pronounces something void ab initio, it means the thing never had legal effect at any moment of its existence. Equally, when an estate or right is said to be good ab initio, it is confirmed as valid from the moment it arose.
The phrase appears most frequently in three contexts:
1. Void instruments. A contract, deed, or marriage declared void ab initio is treated as though it never existed. No legal consequences flow from it, and no subsequent act can ratify it.
2. Trespasser ab initio. Under the common law Six Carpenters' Case principle, a person who enters land or property under lawful authority (such as a license or legal process) but subsequently abuses that authority may be deemed a trespasser from the beginning — retroactively stripping the original lawful entry of its protected status.
3. Invalid proceedings. A judgment, statute, or administrative action that is void ab initio produces no binding legal effect and may be challenged at any time, as opposed to a voidable act that stands until set aside.
Common Language
Modern common usage (Wiktionary): "From the time when a law, legal right, decree, or contract comes into force"; also used in scientific contexts meaning "calculated from first principles without additional assumptions"; and in aviation training contexts, meaning instruction taken with no prior qualifications.
Historical common usage: The phrase has no independent entry in Webster's 1913, reflecting its character as a Latin legal term of art rather than a naturalized English expression.
Editorial note: The scientific and aviation senses of ab initio share the surface meaning of "from the start" but carry none of the legal weight attached to retroactive nullity or retroactive liability. A researcher encountering the phrase in a legal source should not import the neutral chronological sense from these other fields. In law, ab initio almost always signals a normative judgment about the legal status of something from its origin, not merely a temporal reference.
Common Confusion
Ab initio is sometimes conflated with nunc pro tunc ("now for then"), but the concepts operate in opposite directions. Nunc pro tunc is a remedial device that gives retroactive legal effect to an order or act that was valid but improperly recorded or delayed. Ab initio, by contrast, characterizes something as having had a particular legal status — valid or void — from its very creation, regardless of any later ruling. One projects legal effect backward to fill a gap; the other describes what the law treats as always having been true.
The trespasser ab initio doctrine is also sometimes confused with conversion or fraud, but the doctrine is technically distinct: it operates on the legal character of the original entry, not on a separate wrongful act.
Why It Matters in Research
The phrase appears throughout primary sources spanning centuries without definitional change, which creates a false sense of consistency. The operative difference in historical materials is not the phrase itself but the doctrine to which it is attached.
The trespasser ab initio doctrine, sourced to Plowd. 6a and Bl. Comm. 440 in all three dictionary sources here, was significantly narrowed by English and American courts over time. Researchers working in 19th-century property or tort materials may find the doctrine applied more broadly than modern courts would recognize. Do not assume that a historical finding of "trespasser ab initio" maps onto a modern trespass analysis without checking intervening doctrinal shifts.
In contract and marriage law, the distinction between void and voidable is the critical axis. Void ab initio means no legal relation ever existed; voidable means the relation existed but may be annulled. Historical sources — including the dictionaries here — sometimes use void loosely. When a historical source says an agreement is void ab initio, verify whether the relevant jurisdiction and period treated that category as truly void or merely voidable, since the practical consequences diverge sharply: voidable agreements can be ratified, void ones cannot.
In constitutional and administrative law research, the phrase is invoked to argue that a law, regulation, or governmental act exceeded authority from its passage and therefore never had legal force. Courts have not uniformly accepted that every unconstitutional act is void ab initio in this strong sense; some jurisdictions apply a prospective invalidity analysis instead. Materials predating the mid-20th century administrative law consolidation should be read with this in mind.
Historical Dictionary Support
All three sources — Black's (1st and 2nd editions) and Burrill — give essentially identical entries: the Latin gloss "from the beginning; from the first act," followed by the same four stock examples (trespasser ab initio, estate good ab initio, agreement or deed void ab initio, marriage unlawful ab initio), and the same citations to Plowden and Blackstone's Commentaries.
The agreement across sources on both text and citations is unusually close, suggesting a shared derivation from the same foundational authorities rather than independent analysis. This is useful for the researcher as confirmation that the phrase carried a settled, stable meaning in Anglo-American legal usage by the 19th century — but it also means that none of the three sources adds analytical depth beyond the examples given.
What the historical dictionaries do not address: the doctrinal limits of the trespasser ab initio rule, the void/voidable distinction in practice, or the constitutional application of the phrase. For those questions, treatise and case law research is necessary.
Jurisdictional Note
The trespasser ab initio doctrine survives in some American jurisdictions and has been effectively abolished or narrowed in others. The void ab initio characterization of marriages is governed by state family law and varies considerably in both scope and consequence. Researchers should not assume uniform treatment across jurisdictions in either historical or modern materials.