Geiriadur Prifysgol Cymru

Geiriadur Prifysgol Cymru ("The University of Wales Dictionary") is the standard historical dictionary of the Welsh language, aspiring to be "comparable in method and scope to the "Oxford English Dictionary"". Vocabulary is defined in Welsh, and English equivalents are given. Detailed attention is given to variant forms, collocations, and etymology. The first edition was published in four volumes between 1967 and 2002, containing 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.
History.
In 1921, a small team at the National Library of Wales, Aberystwyth, organised by the Rev. J. Bodvan Anwyl, arranged for volunteer readers to record words. The task of editing the dictionary was undertaken by R. J. Thomas in the 1948/49 academic year.
The first edition of Volume I appeared in 1967, followed by Volume II in 1987, Volume III in 1998, and Volume IV, edited by Gareth A. Bevan and P. J. Donovan, in December 2002. Work then immediately began on a second edition. 
In January 2008 Andrew Hawke was appointed as managing editor, following the retirement of the previous co-editors.
On 26 June 2014 a free online version of the dictionary was launched and on 24 February 2016 free apps for iOS and Android were launched.