Monolingual learner's dictionary
A Monolingual learner's dictionary (or MLD) is a type of dictionary designed to meet the reference needs of people learning a foreign language. MLDs are based on the premise that language-learners should progress from a bilingual dictionary to a monolingual one as they become more proficient in their target language, but that general-purpose dictionaries (aimed at native speakers) are inappropriate for their needs. Dictionaries for learners include a great deal of information on grammar, usage, common errors, collocation, and pragmatics, which is largely missing from standard dictionaries because native speakers tend to know these aspects of language intuitively. And while the definitions in standard dictionaries are often written in difficult language, those in a monolingual learner’s dictionary aim to be simple and accessible.
Monolingual learner's dictionaries have been produced for learners of several languages, including German, Spanish, Dutch, and Chinese. But most of the activity in this field is in the area of dictionaries for people learning English. The first English MLD, published in 1935, was the New Method English Dictionary by Michael West and James Endicott, a small dictionary using a restricted defining vocabulary of just 1490 words. Since the end of World War Two, the MLD has been a publishing phenomenon, with global sales running into tens of millions of copies, reflecting the boom in the English language teaching industry.
Probably the best-known English monolingual dictionary for advanced learners is the Oxford Advanced Learner's Dictionary. now in its eighth edition. It was originally published in Japan in 1942 as The Idiomatic and Syntactic Dictionary of English, written by A. S. Hornby and two collaborators. It was subsequently republished as A Learner's Dictionary of Current English in 1948, before acquiring its current name.
Other publishers gradually entered the market. The Longman Dictionary of Contemporary English was published in 1978, and its most striking feature was the use of a restricted defining vocabulary, which is now a standard feature of learners' dictionaries. There are currently six major MLDs for advanced learners. In addition to the Oxford and Longman dictionaries, these are:
- Collins Cobuild English Dictionary, first published in 1987
- Cambridge International Dictionary of English, 1995, now published as the Cambridge Advanced Learner's Dictionary
- Macmillan English Dictionary for Advanced Learners, 2002
- Merriam-Webster Advanced Learner's English Dictionary, 2008
All of these dictionaries are available in hard copy and online.
Since the 1980s, the English MLD has, arguably, been the most innovative area in the field of lexicography, in terms of both the way dictionaries are written and the aspects of language which dictionaries describe. Notable advances include:
- the use of corpora as a basis for language description, a radical innovation which was introduced by the COBUILD project in the 1980s and is now standard practice in lexicography
- the use of intelligent software for extracting information from corpora and for automating the dictionary-making process
- detailed information about collocation, such as the ‘collocation boxes’ in the Macmillan English Dictionary for Advanced Learners, which give lists of high-frequency collocates, identified using ‘Word Sketch’ software 
- the use of digital media: MLDs were among the first dictionaries to appear on CD-ROM, with the Longman Interactive English Dictionary leading the way in 1993. More recently the six MLDs listed above have become available in free online versions, among them the Merriam-Webster Advanced Learner's English Dictionary and the Macmillan English Dictionary for Advanced Learners
Monolingual learner's dictionaries have also been the subject of a great deal of research into how people use dictionaries, and are probably the most intensively-studied type of dictionary.
Online dictionaries 
The Internet offers a range of online dictionary resources. Some, like the Open Dictionary of English, are explicitly designed as learner's dictionaries, and may even include built-in, adaptive tutoring.
Others, like Wordnik or Wikipedia's own Wiktionary, don't have that explicit focus, yet provide useful resources to learners due to their collaborative nature, which tends to produce definitions and sample sentences that are more diverse and written in popular terms.
- Rundell, M., 'Recent trends in English pedagogical lexicography', International Journal of Lexicography, 11/4, 1998: 315-342
- Bejoint, H., The Lexicography of English. Oxford University Press, 2010: 163-200
- Cowie, A.P., English Dictionaries for Foreign Learners, Oxford University Press 1999
- Sinclair, J.M. (Ed.), Looking Up: an account of the COBUILD project, Collins, 1987
- Rundell, M. and Kilgarriff, A., 'Automating the creation of dictionaries: where will it all end?', in Meunier F., De Cock S., Gilquin G. and Paquot M. (Eds), A Taste for Corpora. A tribute to Professor Sylviane Granger. Benjamins, 2011
- Kilgarriff, A. & Rundell, M. Lexical profiling software and its lexicographic applications – a case study. In Braasch and Povlsen (Eds.) Proceedings of the Tenth Euralex Congress, University of Copenhagen, Denmark. 2004, 807-818.
- Nesi, H., 'Dictionaries in electronic form', in Cowie, A.P. (Ed.), The Oxford History of English Lexicography, Oxford University Press 2009: 458-478
- Lew, R., Introduction to Special Issue on Dictionary Use, International Journal of Lexicography, 24/1, 2011: 1-4
The dictionaries themselves 
- Open Dictionary of English, ODE
- Cambridge Dictionaries themselves
- Collins COBUILD Advanced Dictionary
- Longman Dictionary of Comtemporary English
- Macmillan English Dictionary Online
- Oxford Advanced Learner's Dictionary
- Merriam-Webster Advanced Learner's Dictionary
- Monolingual vs. bilingual dictionaries A discussion of dictionary types with regard to foreign language learners.
- Monolingual, bilingual and bilingualized dictionaries for learners A book-length study comparing the effectiveness of monolingual, bilingual and bilingualized dictionaries for learners of English