Someone recently mentioned the "plain meaning" rule in one of my questions about statutory interpretation. Is there an official dictionary - like Merriam-Webster - that judges are supposed to rely on? Or do we assume that any of the leading publishers of dictionaries have essentially identical definitions?

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I don't believe there is a single "standard" dictionary that judges use. Presumably any well regarded and common one will do as long as it can be cited. You can see from Justice Alito's opinion in Intel Investment Policy Comm vs Sulyma that he references several dictionaries.

Although ERISA does not define the phrase “actual knowledge,” its meaning is plain. Dictionaries are hardly necessary to confirm the point, but they do. When Congress passed ERISA, the word “actual” meant what it means to-day: “existing in fact or reality.” Webster’s Seventh New Collegiate Dictionary 10 (1967); accord, Merriam-Webster’s Collegiate Dictionary 13 (11th ed. 2005) (same); see also American Heritage Dictionary 14 (1973) (“In existence;real; factual”); id., at 18 (5th ed. 2011) (“Existing in reality and not potential, possible, simulated, or false”). So did the word “knowledge,” which meant and still means “the fact or condition of being aware of something.” Webster’s Seventh New Collegiate Dictionary 469 (1967); accord, Merriam-Webster’s Collegiate Dictionary 691 (2005) (same); see also American Heritage Dictionary 725 (1973) (“Familiarity, awareness, or understanding gained through experience or study”); id., at 973 (2011) (same).
Thus, to have “actual knowledge” of a piece of information, one must in fact be aware of it. Legal dictionaries give “actual knowledge” the same meaning: “[r]eal knowledge as distinguished from presumed knowledge or knowledge imputed to one.” Ballentine’s Law Dictionary 24 (3d ed. 1969); accord, Black’s Law Dictionary 1043 (11th ed. 2019) (defining “actual knowledge” as “[d]irect and clear knowledge, as distinguished from constructive knowledge”).

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    US courts prefer North American dictionaries over UK or Australian but that’s because there are regional differences in English. If a US court was ruling on a contract under English law no doubt they would use the Oxford and Cambridge dictionaries.
    – Dale M
    Apr 22, 2020 at 12:32
  • @DaleM Ha, yes that would be the case.
    – pboss3010
    Apr 22, 2020 at 12:45
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    Correct. Dictionaries are basically evidence which courts may take judicial notice of, rather than authoritative statements of law, a dictionary is the weakest possible authority for a point, compared, for example, to alternatives like legislative or regulatory definitions, case law, legislative history, legal treatises and law review articles, use in similar statutes, overall context within a statute, and testimony at trial concerning the meaning of a word. Black's law dictionary takes almost all of its definitions from case law.
    – ohwilleke
    Apr 22, 2020 at 18:22

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