A bilingual dictionary or translation dictionary is a specialized dictionary used to translate words or phrases from one language to another. Bilingual dictionaries can be unidirectional, meaning that they list the meanings of words of one language in another, or can be bidirectional, allowing translation to and from both languages. Bidirectional bilingual dictionaries usually consist of two sections, each listing words and phrases of one language alphabetically along with their translation. In addition to the translation, a bilingual dictionary usually indicates the part of speech, gender, verb type, declension model and other grammatical clues to help a non-native speaker use the word. Other features sometimes present in bilingual dictionaries are lists of phrases, usage and style guides, verb tables, maps and grammar references. In contrast to the bilingual dictionary, a monolingual dictionary defines words and phrases instead of translating them. History The Roman Emperor Claudius (10 BCE – 54 CE) is known to have compiled an Etruscan-Latin dictionary, now lost. One substantial bilingual dictionary was the Mahāvyutpatti. The Mahāvyutpatti (Wylie: Bye-brtag-tu rtogs-par byed-pa chen-po), The Great Volume of Precise Understanding or Essential Etymology, was compiled in Tibet during the late eighth to early ninth centuries CE, providing a dictionary composed of thousands of Sanskrit and Tibetan terms designed as means to provide standardised Buddhist texts in Tibetan, and is included as part of the Tibetan Tangyur (Toh. 4346). Dictionaries from Hebrew and Aramaic into medieval French were composed in the European Jewish communities in the 10th century CE. These were used for understanding and teaching the Talmud and other Jewish texts. Challenges The most important challenge for practical and theoretical lexicographers is to define the functions of a bilingual dictionary. A bilingual dictionary works to help users translate texts from one language into another or to help users understand foreign-language texts. In such situations users will require the dictionary to contain different types of data that have been specifically selected for the function in question. If the function is understanding foreign-language texts the dictionary will contain foreign-language entry words and native-language definitions, which have been written so that they can be understood by the intended user groups. If the dictionary is intended to help translate texts, it will need to include not only equivalents but also collocations and phrases translated into the relevant target language. It has also been shown that specialized translation dictionaries for learners should include data that