Dictionary.com Dictionary & Thesaurus - Free
This app is pretty amazing. Frankly I wonder how much space it's taking up on my phone!
Not only do you get a fairly detailed definition of the word you're looking for but the app also give you pronunciation, will speak the word to you (yay for those of us that just can't sound stuff out) and even gives you the origin of the word which is extra awesome. Of all the free dictionary apps this one will likely be the dictionary app of choice.