Merriam-Webster's Dictionary is a free app you can download on your iphone/android to improve your vocabulary.  My students appreciate this app for all of its convenient features.  I always encourage students to keep a vocabulary notebook they can easily access and read to brush up on their vocabulary.  Of course, we all know it is one thing to learn a new word, but to retain this word and actually use it in conversation can be the biggest challenge.  A couple of great things about this app:  There is a save feature (in the shape of a heart) at the bottom of a given word.  When you click on this heart, it automatically saves it to your favorites.  If you click on the 3 bars on the top left hand corner, you will see your list of 'favorites'.  Another great feature is that you can have access to definitions even without internet access.  

Download this app to help increase your vocabulary :).  

Features: 

* Universal App

* New Vocabulary-Building Quizzes: fun, fast quizzes to learn new words or test your vocabulary

* Voice Search: look up a word without having to spell it

* Word of the Day: learn a new word every day.

* Integrated Thesaurus: synonyms & antonyms are included with the dictionary

* Example Sentences: understand how a word is used in context

* Quick Definitions: perfect for on-the-go lookups

* Audio Pronunciations: voiced by real English speakers, not text-to-speech robots

* Favorite Words and Search History: Keep track of the words that are most important to you

* Favorite Word syncing through iCloud: Share your favorites between devices

* Apple Watch Extension: See Word of the Day, look up words with voice search, and keep track of your favorites from your watch

(https://itunes.apple.com/us/app/merriam-webster-dictionary/id399452287?mt=8)