There are plenty of dictionary apps out there, but Ultralingua is for serious wordsmiths. It contains several dozen complete dictionaries with definitions, conjugations, slang, and even translation into multiple languages.
When you install the app, you get a full dictionary and thesaurus in English. This is quite a bit more detailed than the dictionaries you'll find in other apps and web resources with tons of usage notes and examples. The other 35+ dictionaries (including the ones for translation) are available via in-app purchases. Each one is a few dollars, so you can just get the ones you want.
There's nothing special about the design—no flashy animations or widgets. It's just words, words, words, but maybe that's what you need.