The best app for saving vocabulary words

Ive been looking for an app similar to the vocabulary builder on the kindle. This is the best one Ive found.

One feature recommendation- allow the user to add words using the share functionality. If Im reading a book in iBooks or the kindle app, it would be so much more convenient to add new words through the share menu, rather than coping the word and switching apps, adding it, and then switching back to the book.

Keep up the awesome work!

Puzzler5555 about Word Vault: Vocabulary Builder, v2.1