Using public domain works as a base for Wiktionary: Several public domain works can be used for sourcing texts into Wiktionary. Webster's dictionary from 1913 is available in plain text online, convenient for copying and pasting. The Century Dictionary 1911 is available as DJVU online, and has to be manually retyped. Some volumes of Oxford English Dictionary are available as DJVU and have to be manually retyped. There are some other sources in public domain, such as 1811 Dictionary of the Vulgar Tongue available in plain text.
See also Wiktionary:Webster's Dictionary, 1913. Many definitions from Webster’s New International Dictionary of the English Language, 1913, have been imported into Wiktionary (probably, as of August 2020, there are no more, or very few, definitions left to import, but the dictionary does also include etymologies, synonyms, and quotations). See also the template {{Webster 1913}}, which is used for all Wiktionary pages that were imported from that dictionary (as of January 2024 there's a lot - a few thousand)
Available online for search at websters1913.com; available as one huge plain text file from Project Gutenberg #29675.
For a past discussion from 2002 and 2003, see the talk page.
The Century Dictionary, 1911 is a public domain dictionary available as DJVU from www.global-language.com. It can be read with a DJVU plugin, but also without it in a JPEG version. It is in public domain per its being published in the United States pre-1924.
The first fascicle of the Oxford English Dictionary was published in 1884, and it was published in fascicles until completion in 1928. Oxford English Dictionary is a great source of word history.
Some scanned fascicles of Oxford English Dictionary under the title A New English Dictionary on Historical Principles by James A. H. Murray can be found at archive.org, as seen in works by James A. H. Murray. They have been scanned by a person whose letter of intent can be seen, as well as his progress as of March 16 2006. He is scanning those fascicles published in the US before 1924, maybe because in the UK the copyright is extended to author's life + 70 years. There seem to be no plain text files converted using OCR.
The volume 1 of OED, 1884, is also available at Fractionary, starting at OED:1_1, and ending at OED:1_1240.
A number of scans are available through a web interface as of 2012.
The online version is available here using a UK library card number of the correct format (e.g. hertsb123456789).
Thesaurus of English Words and Phrases by Peter Mark Roget is a public domain thesaurus, organized around word clusters. See also {{R:Roget 1911}}.
Available online for search at ARTFL Project; further available from gutenberg #22 and gutenberg #10681.
A 1864 edition available at Thesaurus of English Words and Phrases at Google books.
A 1911 edition available at Google books, as Roget's Thesaurus of English Words and Phrases, edited by C. O. Sylvester Mawson, 1911, Thomas Y. Crowell Company, New York
A 1922 revised edition Roget's International Thesaurus of English Words and Phrases by Christopher Orlando Sylvester Mawson, Peter Mark Roget, Thomas Y. Crowell company, should be in public domain per being published in the United States pre-1923. An online version is available at Bartleby.com.
Moby Thesaurus II is a public domain thesaurus created by Grady Ward, featuring a list of synonyms for each word. See also {{R:Moby II}}.
Allen's Synonyms and Antonyms by F. Sturges Allen, 1921, Harper & Brothers, New York is a public domain dictionary of English synonyms and antonyms, available at archive.org. Its being public domain follows from its being published in the United States before 1924; see also S:Help:Public domain.
A Latin Dictionary by Charlton T. Lewis & Charles Short, 1879, is a PD Latin dictionary. See also {{R:L&S}}. Searchable online in Perseus Project, here "bonus". Available as scanned PDF and browsable online at archive.org, as published at 1891.
WordNet is not in public domain.
Princeton's WordNet® is a semantic network with a byproduct as a dictionary. Its core is synset (a set of synonyms) rather than words, and mainly designed for computers to process NLP-related tasks. Its data is modern, unlike the dated Webster's 1913 Dictionary. Most definitions were written by their lexicographers. Please check out WordNet homepage for more details.
WordNet is available under WordNet 3.0 license. It requires copying the disclaimer to any distribution but does not require redistribution of any modification under any open-source license again. Please read and follow its license requirement carefully. It's not clear whether WordNet 3.0 license is compatible with Creative Commons Attribution/Share-Alike License 4.0 and GNU Free Documentation License (GFDL), currently used by Wiktionary. Refer to Wiktionary talk:Princeton WordNet for more detail.
A Japanese Dictionary by James Curtis Hepburn (1815-1911). Japanese title is 「和英語林集成」 (Waei Gorinshūsei) and English title is "A Japanese and English Dictionary with an English and Japanese Index".
There are several versions (『和英語林集成』各版体裁). Some versions available at Meiji Gakuin University Library Digital Archives (Japanese interface).
The latest version is 第九版 (Seventh Edition) 1910. It available at Google Books.
Scans of the third edition (1886) are available in JPG and PDF from the National Institute for Japanese Language and Linguistics (NINJAL).
Published in London in 1881, the Tibetan–English dictionary by the Moravian Church missionary H. A. Jäschke (d. 1883) is in the public domain and is available from the Internet Archive.
Other public domain sources:
Other sources that are not in public domain: