The best ESL dictionary for teachers, tutors and intermediate to advanced ESL students is the "Longman Advanced American Dictionary." This article describes some of the features of Longman's and how it may be useful to you.
I should mention that this isn't specifically a dictionary for ESL. There are other dictionaries out there that specifically call themselves "ESL dictionaries," such as the McGraw-Hill Education Essential ESL Dictionary.
However, in my experience teaching ESL to adults, the best dictionary for students and teachers is Longman.
Many ESL students start with a bilingual dictionary. Starting with a bilingual dictionary is probably essential for beginning and even intermediate learners of English as a Second Language or English as a Foreign Language (EFL), especially when they are doing homework or studying English on their own.
However, for advanced level ESL students, an English-only dictionary is absolutely essential to master the language.
Longman is the best ESL dictionary for English language learners because it provides definitions that are commonly used today.
The problem with many English dictionaries is that they either provide too many definitions (including definitions that are no longer used), in which case the ESL learner may not be able to determine the best definition; or the books are too small and don't provide enough definitions.
Longman Advanced American Dictionary provides sentences as examples for many words. It tells you when a word is commonly mistaken with another word. It provides lists of spoken phrases, lists many phrasal verbs with definitions, and it has many grammar usage notes to help English language learners with word choice. There's a separate section with "Language Notes" on prepositions, adjectives, idioms, modal verbs, phrasal verbs, articles, etc.
These unique features are extremely helpful for teachers and students alike.
I believe this English dictionary is the best for
advanced-level English learners and I recommend it to all of my
students. Every student I've recommended this dictionary to has spoken
highly of it.
Teachers will also find it a useful dictionary for teaching ESL. It saves time by zeroing in on the best definitions and the usage notes help to answer many questions that students ask. I use mine with my students regularly.
Longman publishes several dictionaries. Be sure to get the one with the exact title above. It comes with a CD-ROM. You can get it in hardcover or paperback.
For more information about ESL books and textbooks, see my ESL Textbook Evaluation article.
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You are free to use my materials in your classes.
I just ask that you include my website address on handouts.