In alphabet books and alphabetical lists, X is always for X-ray. Or sometimes xylophone. Which isn't very imaginative when it comes down to it, 'cause there are a bunch of much cooler words that begin with X. Xiphoid, for instance: a particular part of the sternum. Here are some other X-words, along with loose definitions and information about certain X-roots. (Note that some of these are from Mrs. Byrne's Dictionary, which though entertaining is not always to be trusted; and I've taken some liberties in my paraphrases of definitions.)
The prefix xantho- means yellow:
xanthochroi: pale-skinned fair-haired people. As in "Gentlemen prefer xanthochroi."
xanthodont: someone who has yellow teeth.
xebec: a particular kind of ship.
The prefix xeno- means stranger or foreigner:
xenogenesis: when an offspring doesn't resemble its parents.
xenograft: grafting from one species to another. Like in the movie The Fly.
xenotropic: I'd expect it to mean "moving toward the strange" (or just "becoming strange"), but apparently it really has to do with organisms that reproduce in other organisms' cells.
The prefix xero- means dry:
xerophagous: eating only dry food. Like a pet on a diet.
xerothermic: hot and dry. "Yeah, it's hot today, but at least it's xerothermic."
xu: a Vietnamese coin.
The prefix xylo- has to do with wood:
xylography: engraving/writing on wood. Like carving initials in a tree or on a desk.
xylophone: an instrument that makes sounds using wood.
xyster: a bone-scraping tool.
(An entertainment: mix and match x- prefixes and suffixes. xenophone: a strange musical instrument. xanthophagous: eating only yellow food. xerodont: one who has perpetually dry teeth. xanthothermic: the yellow equivalent of hot pink.)
And then there are lots of proper names, of people and places, that begin with X: Xanadu, Xanthippe, Xanthus, Xavier, Xenophon, Xerxes, Xhosa, Xiamen, Xingu, Xizang (aka Tibet), and so on. (Xingu, besides being a tributary of the Amazon, is also the name of the minstrel in James Thurber's marvelous The Thirteen Clocks, whose "name begins with X, and doesn't.")
Also, X begins a bunch of scientific (mostly chemical and botanical) terms, many of them based on some of the roots noted above.
In all, there are over 70 words that begin with X. ...Only of course that depends on how you count them. There are over 70 main entries in Merriam-Webster's Collegiate Dictionary, Tenth Edition that begin with X, but does each of those entries really count as a "word"? Prefixes get their own entries; the word x gets two separate entries, one as a noun and one as a verb; not to mention X, which gets its own entry as a movie rating. Then again, there are plenty of forms of X-words that don't get their own entries: xenophilous, for instance, is listed as part of the entry for xenophile.
A reader recently asked me how many words there are in the English language. It's an interesting question, but not subject to easy answers any more than the question of how many words there are that start with X. A first approximation might be looking at a good dictionary and finding out how many main entries they list. (Dictionaries sometimes include articles explaining what words they count when they're making claims about how many words the dictionary contains. They generally count only main entries.) MW10 claims to contain "[m]ore than 215,000 ... definitions," but of course many words have more than one definition. I've seen suggestions that MW10 contains about 160,000 main entries, which seems like a reasonable approximation of "the number of words" listed in that dictionary.
For a more accurate count of the number of words "in English," you can look at an unabridged dictionary. Even unabridged dictionaries may not contain advanced technical vocabularies in all fields of human endeavor—there are an awful lot of specialized vocabularies out there. Still, the number of main entries in an unabridged dictionary should give a decent approximation of a reasonable claim about "the number of words in English." Merriam Webster's Third New International Dictionary (from which, I believe, MW10 is abridged) contains "[n]early a half million entries."
On a tangential note, remember the old (false) idea about Eskimos having a hundred (or however many) words for snow? A while back, Dominus suggested a game based on it:
Assume that if a language has a large number of words for a particular concept, that does imply that the concept is very important to the culture. Think of a concept that your language has a large number of words for. "Draw conclusions (in-?)appropriately." For an example, Dominus listed these English words: aloof, arrogant, conceited, contemptuous, disdainful, egotistical, haughty, hubristic, ostentatious, overbearing, proud, scornful, self-important, self-satisfied, smartass, smartypants, smug, sneering, snobbish, snooty, supercilious, superior, vain. (That's not an exhaustive list.) So Americans have twenty-five words for "smug"!
This game could of course be played with a thesaurus, but it's probably more fun without one. Note that you get to decide for yourself whether words are close enough in meaning or connotation to count; looser relations means more words on a given list, which means more possible entertaining conclusions about the culture.