One overlooked feature in iOS 5 is the built-in dictionary.
Huh? Yea, we thought the same thing. Apple included a dictionary in iOS 5, allowing users to quickly look up the definition of a word from nearly anywhere in the OS.
To get the definition for a word, simply highlight the word and tap on Define. If Define isn't an option, there should be an arrow on the right side of the pop-up, tap on it, and then press Define. Doing so will launch a dictionary, providing you with a definition, all the while not forcing you to leave the app you're in.
Watch the video for a more thorough rundown of how it all works.