Here is the list of the best dictionary apps for iPhone and iPad. You can see top and powerful dictionary and translation apps for your iOS devices.
English dictionary apps for iPhone & iPad
1. WordBook - English Dictionary and Thesaurus ( iPhone )- This is one of the most widely-used iPhone dictionary apps. WordBook dictionary come with many features such as- pronunciation for every single entry, spell checker with pop-up spelling suggestions, bookmark and note management, 70,000 usage samples, 23,000 root word origins, 150,000 entries, 220,000 definitions, simple user interface etc. that make it very useful.
2. Oxford Advanced Learner’s Dictionary, 8th edition ( iPhone + iPad )- One of the best dictionary apps available on the App Store. Oxford dictionary helps you to know words meanings and improve your English skills. Oxford dictionary app also teaches you how to say or use any English word (with pronunciation, American & British). If you're looking for serious and powerful dictionary app for your iPhone and iPad, Oxford Advanced Learner's Dictionary is one of the best options for you.
3. Concise English Dictionary & Thesaurus ( iPhone )- If you're looking for an inexpensive, easy to use but powerful dictionary app for your iPhone, use Concise English Dictionary app. This light and fast dictionary app has a very clean user-interface and it comes with so many features like- 591,700 entries, 4.9 million words, offline use, 134,000 pronunciation guides, audio pronunciation ( British, American & Australian ), editable history and bookmarks feature and more. 
4. WordWeb Dictionary ( iPhone + iPad )- A very useful and popular English dictionary app that is available for free on the iOS App Store. This universal iOS dictionary app comes with 285,000 words, 225,000 word sense definitions, 85,000 text pronunciations, 70,000 usage examples, synonyms, similar and related words features, offline use and more. You cannot easily get these features on any other free (no ads) English dictionary app for iPhone and iPad.
5. Advanced English Dictionary & Thesaurus ( iPhone )- As its name indicates, it is really an advanced English dictionary app for iPhone. This is a popular and one of the most comprehensive dictionary app for iPhone that comes with 4.9 million words, 591,700 entries, 335,000 relations, audio pronunciation(US, UK & Australia) etc.
6. Dictionary.com Dictionary & Thesaurus for iPad ( iPad )- A popular and well-designed English dictionary app by Dictionary.com for your iPad. This dictionary app works without Internet connection and helps you to expand your vocabulary and learn how to use them. This is one of the best dictionary apps that are available for free and especially designed for iPad.
7. Merriam-Webster Dictionary & Thesaurus ( iPhone )- If you're looking for the best American English dictionary app for iPhone, use Merriam-Webster dictionary app. This is one of the best American English dictionary apps available on the App Store.
8. Dictionary!! ( iPhone + iPad )- This popular and full-featured iOS dictionary app that helps you to expand your vocabulary and improve your English skills. You can translate English into more than 40 languages offline, play word games, read articles and share words (via social networking sites, email, and text) with this dictionary app. If you're looking for a fully-functional and easy to use dictionary app for your iPhone and iPad, this app is for you.
9. iTranslate - translator & dictionary - translate 80+ languages ( iPhone + iPad )- If you're looking for a free and powerful dictionary+translation app for your iPhone and iPad, this is the right option for you. This is one of the best multilingual(more than 80 languages) dictionary and translation apps for iDevices. This fully-functional dictionary+translation apps has a very clean user interface that lets you use this app very easily and quickly. It also lets you copy words and paste any in any app and share words using Facebook, Twitter, Messages and Mail.
Foreign language dictionary and translation apps for iPhone & iPad
10. English-Spanish Reference Dictionary ( iPhone + iPad )- This is one of the best English to Spanish dictionary apps for iPhone and iPad. It works offline and comes with more than 98,000 definitions, 40,000 examples, 718,000 entries, 524,000 translations, 558,000 synonyms and other useful features that make it a complete all-in-one English to Spanish dictionary app for iOS devices.
11. French English Dictionary and Translator (Le Dictionnaire Français - Anglais) ( iPhone + iPad )- If you are looking for a powerful French to English dictionary and translation app for your iPad or iPhone, this app is for you.
12. Japanese ( iPhone + iPad )- Looking for a full-featured Japanese dictionary app for your iPhone and iPad? You can use this dictionary+translator app on your iOS devices. This Japanese dictionary app comes with more than 174,500 entries and 52,000 examples and works offline.
13. German English Dictionary and Translator (Das Deutsch-Englische Wörterbuch) ( iPhone + iPad )- This is one of the most popular German To English dictionary+translator apps for iPad and iPhone. It has a clean & beautiful user interface and many useful features, such as- 280,000 translation pairs, integrated Wikipedia and Bing/Google Translate, automatic language detection in search mode and more.
14. Pleco Chinese Dictionary ( iPhone + iPad )- This is a very useful Chinese dictionary app for learning Chinese. If you're looking for a useful, functional and free Chinese dictionary app for your iPhone or iPad, you should use this dictionary app.
15. Portuguese English Dictionary Box, Offline Translator, Pronunciation, Image Search & Flashcards / Dicionário Inglês Português & Tradutor ( iPhone + iPad )- If you are looking for the best Portuguese/English dictionary+translation app for your iPhone and iPad, use this app. This dictionary app has a very clean UI and all the features you expect from a full-featured Portuguese dictionary app.