Legal dictionaries — what they are, and what they aren’t
A legal dictionary is a reference book. At its best it is authoritative and precise; courts do sometimes cite one as evidence of a word’s ordinary or technical meaning. But it is persuasive at most — never binding. It does not make law, confer rights, or override a statute or a court’s decision. Keep that distinction and a dictionary becomes a powerful tool; forget it and you’ll build an argument on sand.
Three dictionaries worth knowing
The most-cited American legal dictionary. Useful, authoritative as reference — but still a dictionary, not law. Early editions (1891, 1910) are now public domain; current editions are in copyright.
The first great AMERICAN law dictionary — written specifically for "the Constitution and Laws of the United States of America," not English law. Public domain. Especially valuable for 19th-century American usage.
Not a law dictionary, but the landmark English dictionary of the era — a primary source for the ORDINARY meaning of words at the time of the Founding. Public domain. Cited by the Supreme Court for founding-era meaning.
The key use: founding-era dictionaries for constitutional interpretation
The single most powerful, legitimate use of a dictionary in law is reading a constitutional word as it was understood when the Constitution was written and ratified — its original public meaning. For that, the right tool is a dictionary from the founding era (Johnson’s 1755; later Webster’s 1828), not a modern one. The Supreme Court did exactly this in District of Columbia v. Heller, 554 U.S. 570 (2008), using founding-era dictionaries to fix the meaning of “arms” and “bear arms.” See the founding-era-meaning page →
Related: our own American legal lexicon · a directory of dictionaries, incl. per-country glossaries · using Black’s Law correctly.