Definition
In old English and Law French legal usage, "sei" functioned as a pronoun meaning "him," "his," or "them." It appears in medieval legal formularies and writs as a function word, not as a substantive legal concept. Its significance is grammatical rather than doctrinal: it structured the meaning of Old French and Latin-influenced legal phrases in a period when Law French was the operative language of English pleading and judicial records.
Common Language
Modern common usage (Wiktionary): An initialism for "solid electrolyte interphase," a term from electrochemistry with no legal application.
Historical common usage: No relevant entry in Webster's 1913; the word does not appear as an English common-use term.
Editorial note: The modern technical meaning of "SEI" as an electrochemical initialism shares nothing with the legal usage. Researchers encountering "sei" in historical legal manuscripts should treat it as a Law French pronoun, not an abbreviation or technical term.
Why It Matters in Research
Researchers working in medieval English legal records, plea rolls, or early treatises will encounter "sei" embedded in Law French passages where it performs a purely grammatical role. Misreading it as a substantive legal term, or conflating it with an abbreviation, will distort the meaning of the surrounding text. Because Law French was heavily abbreviated and contextually compressed in manuscript sources, recognizing "sei" as a pronoun is a basic decoding skill for anyone navigating Year Books, early writs, or records from the period of Glanvill and Bracton.
The word appears in close proximity to entries concerning sedition and treason in Burrill's treatment, but this is a matter of alphabetical arrangement rather than conceptual connection. Do not read doctrinal significance into that proximity.
Historical Dictionary Support
Burrill's Law Dictionary is the sole shelf source for this entry. Burrill identifies "sei" as a Law French pronoun — "him; his; them" — with attribution to Kelham's Law French dictionary. Burrill's placement in the same passage as references to Glanvill and Hale's Pleas of the Crown reflects alphabetical coincidence with neighboring entries on sedition, not any substantive link between "sei" and those doctrines.
No other historical legal dictionary in the Law Mind corpus provides a substantive entry for "sei" as a legal term of art. The word's legal utility was entirely mechanical: it gave Law French sentences their pronominal structure. As Law French declined and was formally abolished in English courts by the Proceedings in Courts of Justice Act 1731, "sei" lost any practical legal application entirely.
Jurisdictional Note
Relevant only to English legal history. No common law jurisdiction retained Law French as a language of pleading after the eighteenth century, and the word carries no operative meaning in modern American, Canadian, or Commonwealth legal practice.