Oxford Dictionary of English
The Oxford Dictionary of English was Countdown's source dictionary used to judge words from Series 43 to Series 70. Each updated edition typically added a number of new words, removes a few and clarifies the validity of some inflections (see Letters round rules). It was replaced by use of the Premium version of Oxford Dictionaries Online (ODO) on a laptop in Series 71, which also removed the pencam. The third and last edition remains visible on the host's desk and part of the Countdown goody bag to this day, and is also still in use on 8 Out of 10 Cats Does Countdown.
Countdown has always favoured shorter editions over the various comprehensive tomes issued by Oxford, both for reasons of television convenience and in an attempt to reward anagramming skills rather than knowledge of rare or obsolete words. Nevertheless even the concise dictionary includes a huge number of words which are likely to be unfamiliar to any one person, and some of these have become popular favourites on the show, such as TANGELO, LEOTARD and FANTOD. In the 2000s, some of the most successful players of this era, such as Conor Travers and Craig Beevers, have taken knowledge of the high-probability obscurities to new heights. Stewart Holden admits that his Grand Final win over Steve Graston hinged on his spotting the word WALDOES, which he had learnt only for its probability.
Susie Dent, the programme's longest-serving lexicographer, has frequently suggested that the words seen on Countdown contribute to decisions made about what to include in future editions. For example, after many years of being disallowed, RESOLE was finally introduced with the ODE 2nd Edition. Popular requests included MOANIEST and clouter ☓ (although these have the anagrams AMNIOTES and COULTER respectively).
Originally, the show began with the Concise Oxford Dictionary and continued to use updated editions of it until the disaster of the 10th Edition, in which many compound words were removed if their meanings were considered from their constituents. After a contestant lost a game after having ROADSIDE disallowed, it was realised that this sort of dictionary was not suitable for the letters rounds. The next series switched to the New Oxford Dictionary of English, and its updated editions were used until the end of Series 70.
Oxford Dictionaries Online
From the beginning of Series 71 in June 2014, the paper dictionary and pencam was replaced with a laptop. The validity of words is now determined using the Premium version of Oxford Dictionaries Online (ODO). A free version of ODO, branded as 'Lexico' since  is cross-referenced by the show's director if a word is invalid according to ODO Premium (this is not necessarily seen in the episode's final cut). If the word is valid according to Lexico, then it is accepted. While both dictionaries operate a slightly different search logarithm, the Premium version of ODO can only be accessed via an annual subscription fee.
Only the British English section of ODO Premium and the UK Dictionary section of Lexico are used for adjudication. The UK Dictionary and US Dictionary sections were originally not separate in Lexico, but as of 2020 this is no longer the case. This means one-word entries in US English and US Dictionary which are otherwise specified as two separate words in UK English (e.g. outearn ☓) are no longer allowed. 
If a word can be found by searching for it under the British English section of ODO Premium, and "Entry from US English dictionary" appears underneath the word's phonetic spelling, it will be allowed; an example of this is COQUI. These cannot be found under UK Dictionary in Lexico. Premium is therefore more voluminous than Lexico, and also more accurate as it explicitly labels some mass nouns as a count noun, and some verbs ending with -ING "as noun".
ODO is updated with thousands of new words four times per year. In order to keep in sync with ODO and dictionary rules, the Apterous website's dictionary is updated quarterly by including and excluding words which would or would not be considered allowable on Countdown.
Dan McColm, the first series champion to be crowned in the ODO era, came up with very modern words included in the dictionary such as WHATEV, and in his last game of his octochamp run spotted that the conundrum BOBHOPTOM became PHOTOBOMB, a word that trended highly in 2014. Elliott Mellor, Luke Johnson-Davies and Sam Cappleman-Lynes have also been noted for finding obscure ODO words that could otherwise not be found in ODE.
- Concise Oxford Dictionary
- Oxford Dictionary of English
- Oxford Dictionaries Online
- Premium version in Dictionary Corner, free version (Lexico) as back-up source - Series 71
- Oxford Dictionary of English on Wikipedia.