X-Com 

Game series, based off of UFO lore as well as own made up stuff. Original came out in 1994, with Terror from the Deep coming out in 1995, and Apoclypse in 1997 as direct descendents. There are other games based in the same universe, as well as two planned but cancelled games named Genesis and Alliance.
PC Gamer has the original free in its 2000 september disc.

As a word it can be used to describe any type of alien defense force, as X-Com comes from a 3 word phrase of close meaning.

You think most X-Coms are pretty crazy, don't you?

X-Com by Material Defender December 8, 2005

X's and O's 

The REAL definition behind X's and O's is this
The first mention in literature of XXX for kisses at the bottom of a letter was in 1901, according to the Oxford English Dictionary. The X itself is very old.

The custom goes back to the early Christian era, when a cross mark or "X" was the same as a sworn oath. The cross referred to the cross of Calvary and the first letter of the Greek word for Christ, Xristos.

Even as little as a hundred and fifty years ago, not many people could read or write. The "X" at the bottom of a document took the place of a signature. They would kiss the "X" as a crucifix or bible was kissed to emphasize the importance of the mark. It was this practice that lead to the "X" representing a kiss.

O is an American thing and represents the arms wrapping around someone.

the person above me believes that everytime a girl signed a letter O that he was getting a kiss, which is why he'll never get laid. X's and O's

X's and O's by Abennobashii May 13, 2008

x-matronic 

1) witty, because the word witty just aint witty enough
2) something dope as fuck
3) a filler word when you cant find the word you want to use

Aes Rocks rymes are fuckin x-matronic

x-matronic by jackdan October 3, 2006

X-Men Unite

Said between friends when they coincidentally say the same thing at the same time. Kind of like jinx, without the bad luck. Friends also cross their arms to form an 'X' while saying the phrase. It's taken from the movie X3: The Last Stand.

My friends and I often think similarly, which leads to many X-Men Unite moments.

X-Men Unite by el greco December 13, 2008

X-Philes 

The Name for Fans of the Best Show Ever Made - The X-Files.

X-Philes are not obsessive.... they're dedicated.

X-Philes by Shipper July 28, 2008

x_xiaos 

the best person ever

x_xiaos is the best person to exist

x_xiaos by kissingxiao February 16, 2022

X-factor 

It's Just that "thing" that makes an individual stand out. Captures your full attention. Side Effect- People are dumbfounded when in the presence of the X-factor.

Yah you got swag cause its easy to jock but i got that X-factor that has no mold only the best acquire it. All original parts no copies.

X-factor by MsMendez May 9, 2009