Hans Wehr is undoubtedly the best know Arabic-English dictionary.
Now in its fourth edition, this is a comprehensive dictionary of modern Arabic, up to date and very well laid out. Any student, whatever his or her level, will find it sufficient if he or she is interested solely in Modern Standard Arabic (MSA), such as is found in newspapers, the Arab media and most books by modern authors.
As the title suggests however, it is strictly a dictionary for modern Arabic, compiled from a large Arabic source material of modern Arab newspapers, books etc. As such, a student who wishes to make a serious study of Classical or Quranic Arabic will it severely limiting.
Hans Wehr is replete with “errors” in definition and usage from a classical Arabic viewpoint, omits many classical Arabic words which have fallen out of use and includes (without comment) many neologisms not yet approved by the Arabic Language Academies, and even what would once have been considered colloquial Arabic.
For example, the dictionary gives two broken plurals for the adjective kathīr (‘many’), namely kithār and kuthur, both of which are in fact colloquial – the only classical plural is the Sound Masculine Plural kathīrūna.
Again, the dictionary gives ‘yesterday’ as one of the meanings of the Arabic word al-bāriḥa. This is again a colloquial usage, arising from the classical al-bāriḥah, meaning ‘last night’.
All of this is perfectly in keeping with the dictionary’s methodology, as these words have become part of the standard, educated Arabic language, and are therefore regarded by the dictionary as correct modern Arabic.
Nonetheless, for the reasons outlined above, this is an essential dictionary for any Arabic student, and he or she will continue to find it useful throughout his or her Arabic education. Even if you are primarily interested in classical Arabic, Hans Wehr, with its focus on modern language, will often help you find just the right way to translate a classical term, where Arabic-English dictionaries which focus on classical Arabic might give arcane English translations.