This iOS One-Tap Dictionary is the best there is and it's free

This is the simplest dictionary in the mobile market today. I've tried a lot of different dictionary apps and I was quite disappointed in the range of selections. There are those that can be used offline or online, but you would need to switch to the app before you can use it. With this new Tap-Dictionary, all I have to do is tap on the word I don't understand and it will immediately show the definition for that word without leaving your current app or screen or browser page. Sure, this means that you can only use it when you're online but having the data pop-up instead of switching apps outweighs that. It supports 56 different languages as of now and more may come.