Meanings of the Letter X, Esoteric and Otherwise
X is a mark, an indicator, an identifier - one leaves their X mark as the signature of an illiterate or incapacitated - indicates where to sign on a form - marks the checkbox, the ballot.
X = 10 (ten) in Roman numerals - examples: "Mac OS X" and also it's used as a mark for scoring a strike in bowling where it has a value of ten
X represents 24 in arcane use as the 24th letter of the alphabet.
X is a symbol for multiplication.
In a coordinate system, the x-axis is the horizontal of the x, y and z.
X indicates added dimension, "by" as a 2x4.
In Latin, "ex" means "out of," as in Greek the preposition "ek" is "motion proceeding out from."
The state of being in the form of an X is called "decussation," also an instance of crossing or the condition of being crossed in the form of an X.
X marks the spot on a map, the final destination as treasure on a treasure map - map key symbol for a mountain - a defensive player in a football diagram
X is the symbol of the sun god, a simple rayed sun or star - Osiris - Rx is Horus Eye, pharmacolgical prescription
Although a pyramid is usually perceived as it is viewed from the side, with triangular faces, it forms an X in a box when viewed from above. An obelisk viewed from above also presents an X in a box because it is capped with the benben pyramid. A pyramid with a separate capstone can be considered a pair of them. This attribute that presents an X from the heavens may have something to say about why the X is a sign of the sun god.
X is a Christogram, a symbol representing Christ as in Xmas, derived from the Greek letter chi that begins the word "christos."
In the old Semitic languages, X represented the cross
X signifies betrayal as to cross someone - XX = double cross
X is a cross by its shape of two crossed lines - to "X out" is to cross out
The crosshairs of a scope used for sighting a firearm forms an X or plus sign.
X is known as the tumbling cross or Cross Saltire, a rotated plus sign symbol of the Red Cross logo or Cross of St. George.
X is a generic counter symbol indicating relative strength - traditionally indicates the strength of an ale (Dos Equis) - XXX moonshine: moonshiners used an "X" to notate how many times a particular batch ran through the still - extreme XXX porn
X identifies one in a series as a placeholder: Gen X or Generation X or Planet X
X represents the unknown, like an implied question mark, whether person, place or thing, marks a thing with mystery - X-Ray - The "X Factor" show refers to the undefinable "something" that makes for star quality - X Games - X Files - Malcolm X - X-Men - Professor X (Xavier).
As the placeholder it represents the generic: Brand X
Generation X is a term for what was then kind of undefined, unknown or more generic. "The photographer Robert Capa coined the term. But Douglas Copeland popularized it with a successful novel published in 1991. "Generation X" tells the story of members of an uncertain and lost generation." (www.dictionary.com/e/x/)
X in genetics: X chromosome - graphical segment of double helix
X means cross as in crossing a boundary, crossing over or "trans" as transdimension or transformation - X-men are transformed - Professor X (Xavier) - 
("An X is an ancient symbol for change or transformation...long associated in medieval and renaissance art with the coming of the Messiah who shall make all things new." -Jim Tresner, 33rd degree Scottish Rite Journal)
Together with O, the X is a symbol associated with the mark of the beast: XO or OX
X with O: X is a kiss as in XOXOXO (O is hug) and indicates opposing team's marks in tic-tac-toe square
(Microsoft's XBox game console is abbreviated from the DirectX box, from Microsoft's proprietary DirectX programming interface. See here for decoding the brand as a pyramid)
X expresses the union of two things, as crossed swords or keys in heraldry, which testifies of alliances, and oaths and secrets kept between two people or a person and an organization or a god.It also has the connotation of sexual union, the crossing of bloodlines, or also the union of species or dimensions.
The X can symbolize the anus, the portal of transformation in ritual or key of David sodomy.
X is a shorthand for any ex word - extra (XL size) - In aerospace, X stands for "experimental."
X=ex=former
X is a symbol of negation or a null - the practice of crossing fingers hidden behind one's back to undo or negate the promise being made - in baseball a strike is marked with an X
X as the terminus - marked as to delete, cancel or blot - to close (dialog box or window) - marks the end of a conversation - X's for eyes indicates death - skull and crossbones is an X and symbol of deadly poison like XXX - sign of a life-death oath "cross my heart and hope to die" - "To name oneself with an X or an ex- is to self-annihilate. It can be destruction for the purpose of creation. A phoenix rises out of the X's. A person reborn as an identity without limitation, without restrictions, without moral code. That's the transformation imagined by creators of comic-book and movie heroes." (aiga.org/x-for-all-or-nothing/)
The X may be signified by the Illumined by crossing two objects, like their arms or legs, swords, keys, branches, bones (with or without a Jolly Roger style skull) ...
I believe members of secret socities signal by crossed arms or legs how their meanings are crossed in communications, that left means right and things are to be turned upside-down, black means white, yes means no, etc. exchanging lies for truth. They are being duplicitous, speaking with a forked tongue. This is akin to "blinds" (as filtering light or communicated information) which is obfuscation.
Witches cross fingers as focusing and holding magickal or demonic energy on the point of intersection to build for the moment of release. This is where "keep your fingers crossed" for good luck comes from, and also crossing fingers to make a wish. Another example is in Ghostbusters, where streams are crossed in firing supernatural weapons to amplify strength.
X rated is a sign of explicit sexual content and pornography (now NC-17)
X can refer to the rave or dance club drug, Ecstasy. Technically, it's methylenedioxy-n-methylamphetamine. It's also called MDMA, E, or XTC.
X signals Straight Edge in punk culture, a group or ethic. Minor Threat's 1981 song "Straight Edge" condemned drugs and alcohol, giving singer Ian MacKaye an "edge." (en.wikipedia.org/wiki/Straight_edge) X, XXX, xXx, sXe and 24 (X, the 24th letter) or hXe (hardcore punk) - "temperance" "Teetotalism" - Signs mark the body like X tattoos and especially an x on the back of both hands. (Teen Idles SF tour)
Camp X, aka Intrepid Park, was a British training facility for secret agents in Canada during WWII. "The point is that evidence is 500 surfacing from various locations that the U.S. government was carrying out mind-control during WW II. This was done under the auspices of British Intelligence (MI-6) during W.W. II. When Canada entered the W.W. II with Britain, one of the best british secret agent training schools was set up between Whitly, Ont. and Oshawa, Ont. on Lake Ontario's north shore across from the U.S.-Can. border. The site was called Intrepid Park. Agents called it "the camp" or "the farm". Today, the CIA have their own "farm" (Camp Perry). The official British name for this spy camp was British Security Coordination Special Training School No. 103 & Hydra. STS 103. It was also known as Camp X. British SOE (Special Operations Executive) set itself in NYC. Americans from the FBI and OSS went to Camp X for training, as well as the SOE,SIS, BSC, and the Canadian RCMP. The British were already using mind-control at this stage. The connections are endless. " (The Illuminati Formula to Create An Undetectable Total Mind Controlled Slave by Fritz Springmeier and Cisco Wheeler)
XX
Number 20 or 2, Roman numerals or series counter
Double Cross System, a British MI5 counterespionage operation in WWII overseen by the Twenty Committee (20=XX)
The XX, a British indie band
Female chromosomes in genetics in contrast to XY of male.
Dos Equis, a brand of Mexican beer (See here for decoding)
Code 33 (Z=1, Y=2, X=3 ) - a reference to Freemasonry and ritual sodomy. See here and here for more.
The appearance of any double X form in an Occult context may be a secretive embedding of the square and compass of Freemasonry, marking their territory and performing symbol magick.
Within the double X two very important symbol letters appear, the W and X. The W is above, the M below. They are mirrored, as above, so below. The W is the sign of the goddess, as Inanna-Ishtar. The M is male, the beast. W*oman / M*an. Hermaphrodite-Baphomet - female dominant. In a series of 4, XXXX, W and M appear overlapped, at full height.
XXX
Number 30 or 3, Roman numerals or series counter
Code for 666. X is the 24th letter. 2+4=6, so XXX transforms to 666. The value of the Greek alphabet letter Chi is 60. Negating the 0, 60 60 60 reduces to 666.
Represents hardcore porn
Moonshine
See here for more.