A dictionary is a collection of words with certain information attached to each word. The word dictionary is derived from the Latin word dictionarium, initially meaning a collection of expressions and idioms. A monolingual dictionary contains descriptive information on the word, giving an explanation of what the word means, often including synonyms. A bilingual dictionary puts the information focus on the translation of the word, mostly also including pronunciation and grammar for the word. Usually dictionaries are organized alphabetically by the so called headword or stem of the word. The headword, also referred to as lemma, is used widely to determine the size of a dictionary. As many languages allow combining words or adding a prefix to create new words using the stem of a word gives a fairly good estimate of the comprehensiveness of a dictionary.
The world’s largest monolingual dictionary
It is believed that the monolingual Dutch dictionary Woordenboek der Nederlandsche Taal is the largest monolingual dictionary available. It took 134 years to finish, starting in 1864 and finishing only in 1998. Initially published in 40 volumes, it is now available online at http://gtb.inl.nl/ The dictionary contains hundreds of thousands of headwords and over a million quotes from sources. Other large monolingual dictionaries include the German dictionary Deutsches Wörterbuch (available online at http://germazope.uni-trier.de/Projects/DWB) and the English dictionary Oxford English Dictionary with about 600.000 entries (most content available online at http://www.oxforddictionaries.com/). Starting with the rise of the Internet many websites such as wordnik.com have started to collect words in an automated way, often exceeding millions of words.
The world’s largest bilingual dictionary
The Ethnologue estimates that there are 6,909 known living languages. Based on this estimate there are millions of possible bilingual dictionary combinations. Therefore, there is no one answer. The Muret-Sanders dictionary of English-German published by Langenscheidt includes over 400.000 words and expressions. The Chinese-Japanese dictionary Dai Kan-Wa jiten includes more than 50.000 Chinese characters and 500.000 compound words. Many online dictionaries surpass these numbers by including compound words, flections of words, idioms and sample sentences in their counting.
Multilingual dictionaries offer translations to a word into several languages at once. A dictionary by Gregg Cox lists over 5 million entries in 225 languages. The Logos online dictionary features over 8 million words in over 250 languages. There are also other dictionaries online, like dict.cc or voc.la.
A side note: The world’s smallest dictionary
An English dictionary by David Bryce is considered to be one of the smallest dictionaries in the world. The size is about 27×18 mm and needs to be read with a magnifying glass. The 384 page dictionary contains about 13.000 words.