Kory Stamper, author of the new book Word by Word: The Secret Life of Dictionaries describes three criteria Merriam-Webster uses for inclusion of words like truther, binge-watch, photobomb and the 1,000 other words that make the cut in a typical year.
The first is widespread use, which means a word needs to be used all over the place and in all sorts of different kinds of places. So if a word is used in VIBE magazine and The Wall Street Journal, that's widespread use. Or if a word is used in California and then it's used in South Africa, that's widespread use.
[The] second is sustained use. So a word needs to actually have kind of a long shelf life, because most words come into the language and either don't have much written use for a long time, or they have a lot of written use and then they drop out [of] use for 10 or 20 years. So we like to see some sustained usage, to make sure that it's sort of settled into the language.
Then the third criterion is meaningful use, which means the word has to have a meaning, which sounds patently ridiculous, but there are a handful of words that show up in print that just don't have a lexical meaning.
And if you're not following Merriam-Webster's Trump trolling on Twitter, you should get on it!