Diachronic
Oxford English Dictionary (OED) ['The definitive record of the English language']
Historical Thesaurus of English (HTE) ['This second edition of the University of Glasgow's Historical Thesaurus of English is a unique resource charting the development of meaning in the huge and varied vocabulary of English. It consists of almost every recorded word in English from early medieval times to the present day, all arranged into detailed hierarchies of meaning.']
Examining the OED [an ongoing project at the University of Oxford, UK; 'By exploring and analysing OED's quotations and quotation sources, this research project seeks to illuminate the foundations of the dictionary’s representation of the English language.']
OED Text Visualizer ['Users can type in text of up to 500 words long to see the etymological source of each words (Germanic, Romance, etc) and when it first entered the language.']
Mapping Metaphor with the Historical Thesaurus ['The Metaphor Map of English shows the metaphorical links which have been identified between different areas of meaning. These links can be from the Anglo-Saxon period right up to the present day so the map covers 1300 years of the English language.' The project produced 'Metaphor in the Curriculum' application.]
Online Etymology Dictionary [based on various print dictionaries]
DSL (Dictionaries of the Scots Language) [brings together Older Scots (12C-1700) in A Dictionary of the Older Scottish Tongue and Modern Scots (1700-) in The Scottish National Dictionary]
Historical Thesaurus of Scots ['focuses on two categories that are rich in Scots vocabulary: weather and sports & games']
Green's Dictionary of Slang ['over 500 years of slang from c. 1500 onwards']
Hobson-Jobson: A Glossary of Colloquial Anglo-Indian Words and Phrases ['a historical dictionary of words current in "Anglo-Indian" and on the Eastern trade routes, from the sixteenth to the end of the nineteenth century']
Old English
Bosworth-Toller Anglo-Saxon Dictionary [images of the main volume]
Bosworth-Toller Anglo-Saxon Dictionary [images of the supplement]
Anglo-Saxon Dictionary [by Jonathan Slocum; main volume only]
Bosworth-Toller Anglo-Saxon Dictionary [digital edition; a free smartphone application is available here]
BosworthToller [downloadable for full-text search]
The Dictionary of Old English (DOE) [The compilation has been in progress at the University of Toronto, Canada. Subscription is required. DOE: A to I is now available.]
A Concise Anglo-Saxon Dictionary, Second Edition (1916) [The fourth edition (University of Toronto Press, 1960) is still easily available in paperback.]
A Thesaurus of Old English [a free, electronic version, revised from the print version; see also 'Learning with the online Thesaurus of Old English (TOE)']
Evoke: Exploring vocabularies and the concepts their words evoke [Andreas Thesaurus, Ælfrician Vocabulary, Beowulf Thesaurus, Old English Martyrology Thesaurus]
A Frequency Dictionary of Old English Prose for Learners of Old English and Historical Linguists ['a list of Old English words arranged according to their frequency, covering the whole York–Toronto–Helsinki Parsed Corpus of Old English Prose']
Old English language dictionary
Old English - English Dictionary
Old English to Modern English Translator
Freelang Old English-English Dictionary
Englisc Onstigende Wordboc (EOW)
Old English Core Vocabulary [a page maintained by Christine Rauer (University of St Andrews)]
Glossary from Bright's Anglo-Saxon Grammar
Glossary from 'King Alfred's Grammar' [also available here]
Dictionary of Old English Plant Names (DOEPN) [a project at the University of Graz and University of Munich; registration is required to view the whole datasheet of the entry. See also 'Anglo-Saxon Plant Name Survey'.]
The Épinal-Erfurt Glossary ['An invaluable guide to the earliest written forms of Old English and the Latin culture of seventh-century Anglo-Saxon England'; entries so far in PDF]
Köbler, Gerhard, Altenglisches Wörterbuch (4. Auflage, 2014) [there is also a link to a reverse dictionary of Old English]
Circolwyrde Wordhord: Old English Computer Glossary [by Carl T. Berkhaut (University of Arizona)]
Middle English
Middle English Dictionary (MED) [a must item for any study of Middle English; a new interface and platform was released in 2018; see also Dictionaries at MED]
A Concise Dictionary of Middle English [a classic dictionary edited by A. L. Mayhew and W. W. Skeat; also available on Project Gutenberg]
Glossarial Concordance to Middle English [by Christopher Cannon (Johns Hopkins University)]
A Glossarial DataBase of Middle English: Canterbury Tales [by Larry D. Benson]
Middle English Glossarial Database [focusing on Chaucer and Gower]
Henry Litwhiler, Chaucer concordance ['All words are counted (with normalized capitalization) just as they appear in the various editions that I have brought together.'; see 'About']
Jacqueline de Weever, Chaucer Name Dictionary [an online version of the print dictionary]
Sinan Kökbugur, Middle-English Glossary
The Bilingual Thesaurus of Everyday Life in Medieval England ['contains vocabulary relating to seven domains of everyday life: Building, Domestic Activities, Farming, Food Preparation, Manufacture, Trade, Travel by Water']
A Dictionary of Middle English Cooking Terms ['currently listed are terms used in Forme of Cury and Two Fifteenth-Century Cookery Books']
James L. Matterer, Glossary of Medieval Cooking Terms ['a collection of commonly-used Middle English words & phrases found in surviving medieval cooking manuscripts, with brief modern English translations']
Jenni Nuttall, Middle English Poetics Glossary ['a work-in-progress, a glossary of terms used to discuss Middle English poetics and versification']
Early Modern English
Michigan Early Modern English Materials (MEMEM) ['citations collected for the modal verbs and certain other English words for the Early Modern English Dictionary']
Lexicons of Early Modern English (LEME) ['searches and displays word-entries from monolingual English dictionaries, bilingual lexicons, technical vocabularies, and other encyclopedic-lexical works, 1480-1755'; Part One of the LEME Corpus (1475-1625) is available here]
Robert Cawdrey's A Table Alphabetical (1604) [the first monolingual dictionary of the English language!]
Linda K. Alchin, Elizabethan Online Dictionary ['provides a fascinating insight into the origins and meanings words contained in the plays of William Shakespeare']
King James Bible - Online Study Bible: Bible Dictionary [a concordance is available here]
David Crystal and Ben Crystal, Shakespeare's Words ['an online website in which we could include every instance of a word and which would help users to search through the database easily']
Open Source Shakespeare, Concordance of Shakespeare's complete works
The Shakespearean Glossary ['you will find the meanings of old and unusual words used in Elizabethan England']
Shakespearean Vocabulary List ['a handy list of some of the most common words used by Shakespeare translated into modern English']
Mary Farrington, A Complete Concordance to the Poems of John Donne ['the first ever complete concordance to Donne’s poems, including not only all the English poems, but also the few Latin poems and semi-poetical writings that were published in volume 8 of the Variorum']
Late Modern English
Mitsuharu Matsuoka (Nagoya University), The Victorian Literary Studies Archive Hyper-Concordance [read the review here]
Johnson's Dictionary Online ['Johnson's Dictionary Online is an active digital humanities project that is working to create an online scholarly edition of Samuel Johnson's famous A Dictionary of the English Language.' A new site launched in 2021 has a complete, searchable, transcribed version of the first folio edition (1775).]
Webster's Dictionary 1828 [some data and information in the print dictionary have been removed]
Varieties of English
EDD (English Dialect Dictionary) Online 4.0 [a digitised version of Joseph Wright's English Dialect Dictionary (1808-1905)]
Dictionary of American Regional English (DARE) ['The digital edition features audio, interactive maps, and insights into the DARE Survey.']
DCHP-2, the Second Edition of A Dictionary of Canadianisms on Historical Principles ['an updated edition of DCHP‐1 Online (Dollinger, Brinton and Fee 2013), which was the first digital version of A Dictionary of Canadianisms on Historical Principles (1967 and 1991 editions, ISBN 0-7715-1976-1)']
Dictionary of South African English ['over 4000 uniquely South African words across three centuries, browsable by category'; available on mobile devices too]
Australian National Dictionary, 1st edition ['The Australian National Dictionary (AND) is a dictionary of Australianisms. It includes words and meanings that have originated in Australia, that have a greater currency here than elsewhere, or that have a special significance in Australian history.' The second edition (2016) was published in print by OUP.]
Dictionary of Newfoundland English ['a reasonable rendering of the detail embodied in the original text (= second edition with supplement)']
CoalSpeak – The Official CoalRegion Dictionary
Languages other than English
Anglo-Norman Dictionary [a revised, online version of the Anglo-Norman Dictionary (1977-92)]
Dictionnaire du Moyen Français (1330-1500) [a dictionary of medieval French]
Das Deutsche Wörterbuch von Jacob und Wilhelm Grimm
Vocabularius Theutonicus online [a dictionary of Middle Low German (1100-1500), with Latin equivalents]
Dictionary of Medieval Latin from British Sources [full-text searching requires subscription, but searching by headwords is freely available from here]