As you'd expect, Google likes data. They're pretty much tracking everything that can possibly be tracked, which can be scary, but they also give their users a lot of access to that data. So what's an "Ngram"? Wikipedia defines it as:
In the fields of computational linguistics and probability, an n-gram is a contiguous sequence of n items from a given sequence of text or speech. The items can be phonemes, syllables, letters, words or base pairs according to the application. The n-grams typically are collected from a text or speech corpus.
Basically, Google has provided us with a way to search for certain words or phrases that have appeared in books over a certain period of time, from about 1800 to 2008, then display the results on a graph to show their frequency of occurrence. The first things I plugged in when I discovered this a few years ago were topics related to the books I was writing. This is what the Google Ngram viewer displays for "parallel universe, alternate universe, multiple worlds":
That is awesome. See, physicist Hugh Everett III proposed his "many-worlds" interpretation of quantum mechanics — the idea that a parallel universe exists for every action and decision we make — in 1957. The theory was promptly ignored in the scientific community until around the late 1970s and early 1980s, though it had appeared in science fiction for decades before that and grown in popularity, particularly in television shows like The Twilight Zone and Star Trek, and much later, Sliders. If you've been reading a lot of YA, you've probably noticed an increase in novels about alternate realities lately, and so has Google.
Let's try something even more popular. This is the search for "vampire":
This looks about right to me. Vampires have been around in folklore and fiction for a while before Bram Stoker wrote Dracula in 1897. But it sure looks like it really took off around 2005. Wonder what could account for that?
Okay, one more:
I find this endlessly fascinating, almost as addictive as Wikipedia or TV Tropes (no links, you're welcome). What do you think of this tool? What sorts of terms are you interested in searching for?