Having access to a dictionary is one way to really improve your vocabulary. When my teacher in eighth grade English class told me that, I ignored it, because who has time to stop reading, grab the dictionary from the shelf (or under my seat, in middle school), and look up that word. By the time I was done with that, I’d have lost any meaning in the reading I was doing, anyway. Ugh.
Flash forward to now, and almost every device has a dictionary attached to it. The same is true for Mac OS X, at least the Mountain Lion variety, and it’s super easy to bring up.