Also an abbreviation for Christmas, originally used by Jewish shopowners who objected to using the word "Christ" in their storefronts, so they had signs like "Xmas gifts 50% off"

Though we see it as the letter X (even sometimes say it as "X-mas"), it is actually from the Greek letter "chi" which has a sound similar to the one in words like "Christmas" or "Christ." "X" (chi) has a long history as an abbreviation for Christ from the early days of Christianity. It is the first letter in Christos (in Greek) which was from what "Christ" was derived (meaning "anointed one," "messiah").

From that comes the abbreviations often seen online: "xtians" and "xtianity" for "Christians" and "Christianity" (sometimes used with pejorative intent).

"The X in Xmas is a substitute cross for the Crucifix of Christ"
- Mark E. Smith

From the song 'No Christmas For John Quays' on The Fall's first LP, 1979's Live at the Witch Trials, It also appears in a far longer form as the final track on the live LP Totale's Turns (It's Now Or Never)

The 'John Quays' of the song is a phoenetic substitute for heroin junkies (pronounced juhn-keys - The two actually sound quite interchangeable when Mark sings it), to which the song actually refers to.

However, sid seems to make a more convincing point than Smith...

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When I was younger (So much younger than today) I always thought that the 'X' stood for 'cross', which was the logical progression from 'criss', (i.e. criss-cross, which is not too dissimilar from Christopher Cross) thereby making the word sort of 'criss-mas' by proxy, which is phonetically correct.

It's that time of year again: Storefronts littered with the newest toys, your local Hallmark store touting their shiniest wrapping papers, and not a single parking spot to be found at the mall. Yeah, it's Xmas... oops, I'm sorry, Christmas time again.

As any regular church-goer (and probably quite a few who aren't) knows, around Christmas time, the Protestant and Catholic gung-hos love to talk about how 'X is no substitute for Christ'. There are poems, signs, buttons, bumperstickers, and all manner of what-have-you about this subject, and yet what many don't seem to realize is that, yes, X is a substitute for Christ. Quite a suitable one, actually. So excuse me, but please get over your X.

Hold on Jim Bakker. Stop right there, Billy Graham. Before I hear one more inane sermon about America trying to remove God from our culture, let's look at a few key facts, shall we?

"In words from Greek, x transliterates the 14th Greek letter, xi,... The formal Greek correspondent of x was chi, hence it is used in phonetics to represent a velar fricative like 'ch' in loch."

Basically, this means that in the Greek language, the 'x' would be parallel to 'ch' when it makes a 'k' sound. Thusly, the 'X' so readily replaces 'Christ', as the Greek spelling for it is ΧΡΙΣΤΟΣ, which, when using English characters, is roughly XPICTOC.

Many centuries ago, copies of the scripture had to be hand written; therefore, commonly used terms were abbreviated using the first letter of the word. The original four gospels (the books of Matthew, Mark, Luke, and John) were written in Greek and so the abbreviation of X for Christ remained.

So perhaps in the future, instead of so many Christians being up in arms about the 'X' in Xmas, we could all hear a bit of truth about its origins.

Sources:
http://www.encyclopedia.com/html/X/X.asp
http://www.inq7.net/opi/2001/dec/20/letter_1-1.htm
http://www.geocities.com/lf_loriflower/5/X.html