Eijirō (英辞郎?) is a very large database of English-Japanese translations. Developed by the editors of the Electronic Dictionary Project and aimed at translators, Eijirō is currently one of the most popular dictionaries on the Internet. Although the contents are technically the same, EDP refers to the accompanying Japanese-English database as Waeijirō (和英辞郎?).
The Eijirō project was started by a translator (who wishes to remain anonymous) as a way to keep track of his English vocabulary. Noting the favorable reception it received when he shared it with his friends, he started the Electronic Dictionary Project, a wiki-like structure that allowed for and even encouraged contributions to the dictionary. This resulted in a comprehensive database that grew to include over 1.66 million entries in the fourth edition.
Although commonly termed a dictionary, Eijirō differs from other Japanese dictionaries such as the Kōjien by giving examples the same weight as definitions. While this may result in less accurate definitions on a one-to-one basis, many translators use Eijirō specifically for this purpose, as this structure allows for a more nuanced understanding of the word and accounts for its appearance in commonly used sayings. In addition, the extremely high entry count (approximately 1.66 million entries in the fourth edition, as compared to 560,000 in the third edition of Webster's Dictionary) virtually guarantees the inclusion of new and obscure words.
In addition, an online version of Eijirō is provided free of charge through the SpaceALC Japanese portal.