Expand Your Lexicon With the Oxford Dictionary of English
The Oxford Dictionary of English app, by Handmark, does a tremendous job of capturing all the OED's information, and making it available on your Android phone. In comparison to Merriam-Webster's dictionaries, the OED goes into far greater detail about each entry. You learn not only the common usage of a word as it is used today, but also the word's origins, its previous meanings, and multiple examples of the word in use. The app version preserves most of those features of the dictionary.
The chief difference between the print version of the OED and the Android app is that the Android app has an abbreviated number of examples of the use of each word in sentences.
Though the app has a high regular price, I bought it on sale for $10. Anyone fascinated by the breadth of the English language will find that this OED app quickly repays the investment.