Definition
A Latin term from medieval English legal practice meaning sameness or identity. Used interchangeably with identitas, idemptitas appeared in pleading and procedural records to assert that two things, persons, or subject matters were one and the same — not merely similar, but legally identical. The term carries no independent substantive doctrine of its own; it functions as a label for the concept of identity wherever that concept does formal legal work, such as in abatement pleas, res judicata arguments, or the matching of parties and claims across proceedings.
Common Confusion
IDEMPTITAS vs. IDENTITAS: These are variant spellings of the same concept, not distinct legal terms. Idemptitas appears to be the older or more Anglicized orthographic form, while identitas tracks more closely to classical Latin. Burrill treats them as equivalent and cross-references them accordingly. A researcher encountering one in a historical document should not assume any difference in legal meaning from the other. Both ultimately resolve to the same underlying question: are these two things, in law, the same thing?
Why It Matters in Research
This term is a navigational hazard rather than a substantive concept. Its primary research value is recognizing it when it appears in old English pleading records, writs, and register entries — particularly in the Registrum Brevium (Reg. Orig.) — and understanding that it signals an identity argument, not a separate doctrine to be traced.
A few specific traps for researchers:
First, spelling variation is significant. Medieval and early modern legal Latin was not standardized, and idemptitas, identitas, and even eadem res could all appear in close proximity in the same record set, sometimes meaning precisely the same thing, sometimes with subtle contextual shading. Do not assume a different spelling indicates a different legal concept without confirming from context.
Second, the term's appearance in a source is a marker of era. Idemptitas is the kind of Latinism that belongs to the period of common law pleading under the old register of writs system. Encountering it in a post-Judicature Act English source, or in American sources after procedural reform, would be unusual and worth investigating. Its presence is a reliable indicator that a document originates in or deliberately borrows from pre-reform practice.
Third, the substantive question it labels — identity — remained doctrinally important long after the term itself fell out of use. Modern res judicata, claim preclusion, and party-identity analysis in joinder doctrine are the living successors to the questions idemptitas was used to frame. Researchers tracing the intellectual lineage of those modern doctrines back through the common law may find idemptitas appearing at key junctures in early authority.
Historical Dictionary Support
Burrill's Law Dictionary is the only shelf source in the Law Mind corpus to include this term, and its entry is brief: idemptitas means sameness or identity, is drawn from old English practice, and is explicitly identified as another form of identitas. The reference to Reg. Orig. 104 et seq. points to the Registrum Brevium, the foundational register of original writs, as the primary textual home for the term.
No other major historical law dictionaries in the Law Mind corpus — not Black's, not Bouvier, not Jacob's — include a standalone entry for idemptitas. This absence is itself informative. The term was sufficiently specialized and archaic that even general historical legal dictionaries did not find it necessary to define. Its appearance in Burrill reflects that dictionary's particular attention to Latin terms of art from early English practice. Researchers relying solely on Black's or Bouvier for Latin pleading terminology would miss it.
What the historical sources collectively do not address is any doctrinal evolution specific to idemptitas as opposed to identitas. The two terms appear to have been used without meaningful legal distinction, and no historical source draws a line between them.
Jurisdictional Note
Idemptitas is a term of English common law practice with no reported American statutory or judicial history as an independent term of art. American courts that addressed identity questions in pleading used identitas or, more commonly, simply argued the concept in English without the Latin label.