The preeminent dictionary of the English language is the Oxford English Dictionary (OED), a multi-volume set published by Oxford University Press that costs about a lot. You can get copies of entries from the OED and other good dictionaries from larger public and academic libraries. The OED is also available on CD-ROM and online by subscription through OED Online. Some membership libraries (e.g. NYLI) and academic libraries provide access to the online OED through their websites.
Good alternative sources for authoritative definitions include the Compact Oxford English Dictionary, the OED on CD-ROM, and good 1-volume dictionaries, such as Webster's Third New International Dictionary and the Random House Webster's Unabridged Dictionary.
Internet Options: Miriam-Webster Online is the equivalent of a standard desk dictionary, plus you can also hear the words pronounced. OneLook Dictionaries lets you simultaneously search many kinds of dictionaries including business, scientific and sports dictionaries (among others); if you are affiliated with a college or university, Credo Reference also simultaneously searches several kinds of dictionaries, including art, finance, psychology, and history dictionaries. Symbols.com posts a dictionary of symbols. Note: Similar information is available in The New York Public Library Desk Reference.
To find more online dictionaries, check out the Yahoo! listing for "Dictionaries". See below for foreign language and legal dictionaries. For medical dictionaries, see the separate entry on "Medical Materials."
Foreign Language Dictionaries: You can link to online foreign language dictionaries through Yahoo!. For online translators, see the "Translations" entry. See the "Legal Dictionaries" section, below, for information on English-foreign language dictionaries translating legal terms.
Legal Dictionaries: The preeminent legal dictionary is Black's Law Dictionary (West), which is available in almost all law libraries, followed by Ballantine's Law Dictionary (LexisNexis), which is available in many. Black's is searchable on Westlaw (DI), is on Lexis (2NDARY/BTINES). Also look in Garner's Dictionary of Legal Usage (Oxford University Press) if you need to understand or demonstrate the meaning and proper use of a legal term.
Several legal dictionaries are posted free including FindLaw Legal Dictionary, Nolo's Free Dictionary of Law Terms and Legal Definitions and the Real Life Dictionary of the Law.
Bouvier's Law Dictionary was the preeminent legal dictionary in the U.S. in from the 19th Century until about the 1930's. The current edition is called The Wolters Kluwer Bouvier Law Dictionary. Historic editions are available in many law school libraries, and the 1856 Edition is posted free by the Constitution Society. For more, see Book Review - The Wolters Kluwer Bouvier Law Dictionary, Desk Edition by Holly Lakatos.
Westlaw has a dictionary of tax law.
In addition to standard dictionaries, West's Words and Phrases collects definitions and interpretations of words and phrases from judicial opinions. In addition, to the large stand-alone set, you can find jurisdiction-specific "Words and Phrases" volumes at the end of some West "Digests" (e.g., West's Maryland Digest). You can also search the "wp" field in any case law database on Westlaw (e.g. wp(frivolous /3 claim)). Note: There is a "Words and Phrases" database on Westlaw, but I've been told it drops older cases and doesn't actually provide the whole case summary.
For terms used in contracts, see A Manual of Style for Contract Drafting, published by the American Bar Association.
Foreign-English Legal Dictionaries: Good Spanish-English Legal dictionaries include Dahl's Spanish-English Law Dictionary (W.S. Hein & Co), Butterworth's Spanish/English Legal Dictionary and McGraw-Hill's Spanish and English Legal Dictionary (McGraw-Hill). Dahl's is available on Lexis (LEXREF;SPDAHL). See also the annotated Bibliography at the end of Dennis Kim-Prieto's "Problems with Current English-Spanish Legal Dictionaries, and Notes toward a Critical Comparative Legal Lexicography," 100(2) Law Library Journal 251, 217 et seq.
For French, Dahl's Law Dictionary: French to English/English to French is well known. It is available in print (W.S. Hein) and on Lexis (LEXREF;FRDAHL).
To find other translating legal dictionaries, your best bet might be to search the terms "law," "dictionary," and the name of the country in Worldcat, and/or one of the other online book catalogs discussed in the "Book Stores" entry and/or the online catalogs of Foreign law collections, such as the Columbia and Harvard law libraries. If that doesn't work, you might want to call a translator or translating service and ask for a recommendation.
Medical Dictionaries: See "Medical Materials."
Style Manuals: For legal, in 2010, Ken Svengalis recommended The Red Book: A Manual on Legal Style (West) and The Elements of Legal Style (Oxford) by Bryan Garner. For more recent recommendations, see the current edition of Ken's Legal Information Buyer's Guide & Reference Manual (New England LawPress).