What Makes a Word a Word

According to conventional wisdom, a word is any group of letters that can be found in a dictionary. Which dictionary? Why, the Unidentified Authorizing Dictionary, of course:

'Is it in the dictionary?' is a formulation suggesting that there is a single lexical authority: "The Dictionary." As the British academic Rosamund Moon has commented, "The dictionary most cited in such cases is the UAD: the Unidentified Authorizing Dictionary, usually referred to as 'the dictionary,' but very occasionally as 'my dictionary.'
(Elizabeth Knowles, How to Read a Word. Oxford University Press, 2010)

To characterize this exaggerated regard for the authority of "the dictionary," linguist John Algeo coined the term lexicographicolatry. (Try looking that up in your UAD.)

In fact, it may take several years before a highly functional word is formally recognized as a word by any dictionary:

For the Oxford English Dictionary, a neologism requires five years of solid evidence of use for admission. As the new-words editor Fiona McPherson once put it, "We need to be sure that a word has established a reasonable amount of longevity." The editors of the Macquarie Dictionary write in the Introduction to the fourth edition that "to earn a place in the dictionary, a word has to prove that it has some acceptance. That is to say, it has to turn up a number of times in a number of different contexts over a period of time."
(Kate Burridge, Gift of the Gob: Morsels of English Language History. HarperCollins Australia, 2011)

So if a word's status as a word doesn't depend on its immediate appearance in "the dictionary," what does it depend on?

Defining Words

As linguist Ray Jackendoff explains, "What makes a word a word is that it is a pairing between a pronounceable piece of sound and a meaning" (A User's Guide to Thought and Meaning, 2012). Put another way, the difference between a word and an unintelligible sequence of sounds or letters is that--to some people, at least--a word makes some sort of sense.

If you'd prefer a more expansive answer, consider Stephen Mulhall's reading of Wittgenstein's Philosophical Investigations (1953):

[W]hat makes a word a word is not its individual correspondence with an object, or the existence of a technique of its use considered in isolation, or its contrasts with other words, or its suitability as one component of a menu of sentences and speech-acts; it depends in the last analysis upon its taking its place as one element in one of the countless kinds of ways in which creatures like us say and do things with words. Inside that unsurveyable complex context, individual words function without let or hindrance, their ties to specific objects without question; but outside it, they are nothing but breath and ink...
(Inheritance and Originality: Wittgenstein, Heidegger, Kierkegaard. Oxford University Press, 2001)

Or as Virginia Woolf put it:

[Words] are the wildest, freest, most irresponsible, most un-teachable of all things. Of course, you can catch them and sort them and place them in alphabetical order in dictionaries. But words do not live in dictionaries; they live in the mind.