One of my favorite things about reading ebooks (besides the fact that I no longer have to figure out how to fit 10 books in my carry on) is how easy it is to look up the definition of a word and then get right back to reading. No more encountering a word whose meaning I'm not sure of, realizing my dictionary is on the shelf all the way downstairs, and deciding I don't really need to know what that word means after all. (I stopped pretending to myself that I would look the word up later years ago.)
But did you know you can also look up words in Mail and Safari?
Tap and hold to highlight the word you want defined and a menu will pop up giving you the option to copy or define the word. Tap Define and you will be shown the definition: