I highly recommend this app – Vocabulary Builder by Magoosh Review

This is an excellent app! It is perfect for studying for the ACT and SAT, which is the way I am using it, or also to just expand your everyday vocabulary. One thing that I like about it is that you have to master the words, and if you get one wrong, you will keep reviewing it. The only downside to this app is that there is only one definition per word. It would be helpful with memorization if there were different definitions.
Review by Eika W. on Vocabulary Builder by Magoosh.