Geiriadur Prifysgol Cymru
Geiriadur Prifysgol Cymru (The University of Wales Dictionary) is the principal historical dictionary of the Welsh language, enjoying a similar status to that of the Oxford English Dictionary in the English language.
Work on the dictionary began in 1921 with a small team of paid staff at the National Library of Wales, Aberystwyth, organised by the Rev. J. Bodvan Anwyl as Secretary, who arranged for a host of volunteer readers to record words. The task of editing the dictionary began in the 1948/49 academic year, under the editorship of R. J. Thomas, with the dictionary being published in 64-page parts, ultimately 61 parts in all.
Parts 1-21 were bound together as Volume I (a–ffysur) in 1967. Parts 22-36, edited by Gareth A. Bevan, were bound together as Volume II (g–llyys) in 1987. Parts 37-50, edited by Bevan and Patrick J. Donovan, were bound together as Volume III (m–rhywyr) in 1998. The final draft entry was written on 6 December 2001, after 80 years compilation, and Volume IV, edited by Bevan and Donovan, was published in December 2002. The final entry is "Zwinglïaidd, Zwinglian (adj.)", the three entries relating to Zwingli being the only words to begin with the letter "Z", which does not ordinarily occur in Welsh.
The First Edition contains 7.3 million words of text in 3,949 pages, documenting 105,000 headwords. There are almost 350,000 dated citations, from the seventh century to 2002, with 320,000 Welsh definitions and 290,000 English synonyms.
In January 2002 work immediately began on the Second Edition, as the words beginning with A and B had been compiled to a different, more concise, plan than the rest of the alphabet. Re-editing of "A" and "B" is scheduled to take until 2008. It is intended that the dictionary may eventually be available online and on CD. The Second Edition is, like the first edition, initially being issued in 64-page parts, approximately every five months. Because of the explosion in new terms since work on the dictionary began, the First Edition omits many now-important words in the language, e.g. cyfrifiadur, computer; meddalwedd, software, and cymuned community.
In January 2008 Andrew Hawke was appointed as Managing Editor, following the retirement of the two Co-Editors, Gareth A. Bevan and P. J. Donovan.
Publishing details 
- Complete Set (4 volumes) eds. R. J. Thomas, Gareth A. Bevan and P. J. Donovan 1967-2002, xii+1366, xxxii+925, xxxvi+859, xvi+748, 195mm×272mm pages, University of Wales Press. ISBN 978-0-7083-1806-5.
- Volume I (a-ffysur) ed. R. J. Thomas, 1967, pp. xx+1366, 195×272mm, University of Wales Press. ISBN 978-0-7083-0504-1.
- Volume II (g-llyys) ed. R. J. Thomas and Gareth A. Bevan, 1987, pp. xxxii+925, 195×272mm, University of Wales Press. ISBN 978-0-7083-0981-0.
- Volume III (m-rhywyr) ed. Gareth A. Bevan and P. J. Donovan, 1998, pp. xxxvi+859, 195×272mm, University of Wales Press. ISBN 978-0-7083-1530-9.
- Volume IV (s-Zwinglïaidd) ed. Gareth A. Bevan and P. J. Donovan, 2002, tt. xvi+748, 195×272mm, University of Wales Press. ISBN 978-0-7083-1804-1.
The individual parts of the second edition (11 parts (a-bôn(1)) up to 2012) are also available at £9.99 each or at a one-third discount to regular subscribers.