Merriam Webster, a top-rated dictionary for Apple mobile devices, listed in top free apps
http://www.pcworld.com/article/228928/best_free_iphone_and_ipad_apps_of_2011_....mod_rel
from site: "Look up any word on your iPhone or iPad with the Merriam-Webster dictionary, or check out the word of the day. The app is simple and easy to use, and the spoken search feature can be particularly handy."