Google's unresting laboratories have come up with a database you can use to track words, phrases, concepts through hundreds of years worth of literature. I'm still not convinced that this is useful, but like many another app it's certainly fun in a frivolous kind of way. The Ngram Viewer allows you to search multiple terms within set periods (say 1800 - 1950) so you can compare their rates of usage in a wide range of books.  My early experiments with it haven't proved particularly fruitful, but no doubt the user rather than the technology is at fault.

Wikipedia tells me that “An n-gram is a subsequence of n items from a given sequence. The items in question can be phonemes, syllables, letters, words or base pairs according to the application.” Wikipedia also warns me that “The introduction to this article provides insufficient context for those unfamiliar with the subject” and invites me to improve it. If I knew what an n-gram was I probably would.