Qfyilyi

X-factor refers to:

• a noteworthy special talent or quality, or




• a variable in a given situation that could have the most significant impact on the outcome:
  
  (ODO)

The first definition is now probably the more common, given the popularity of the international talent shows that look to promote artists with the 'X-factor'.

The expression appears to be from the 30's according to M-W; Ngram shows earlier usages , but they appear to be unrelated to the current meanings.

X as a letter, does not seem to carry the connotation of something special or particularly significant among its different meanings. (Collins)

Question:

Where does the expression come from (statistics?, science?) and what did its earliest usages meaning "noteworthy and special talent" refer to?

I reckon that the idea in widespread public consciousness that X means something not only mysterious but extraordinary dates from at least the discovery and naming of X-rays in the late 19th Century.

The letter was chosen as "signifying an unknown quantity" according to Wikipedia. See here for more information, including the original adoption of X and Y as unknown variables by Rene Descartes in the 17th Century.

Once their use in medicine became widespread, their "magical" nature captured the imagination of the general public.

See here for examples of the term being used to market various products unconnected with radiation, from prophylactics to batteries. It was a fin-de-siecle buzz-word.

EDIT -

It doesn't seem a huge leap to apply the letter in other ways. Sorry for the indirect reference:

Tolstoy suggests in War and Peace that "in warfare the strength of an army is the product of its mass and of something else, some unknown factor x."

(from Systems Intelligence in Leadership and Everyday Life , 2007 p.25)

Note that "X factor" has only really meant a "noteworthy talent" since the talent show. In earlier versions of Idol, they had spoken about contestants "having an X-factor", which was basically used equivalent to "having a certain je-ne-sais-quoi".

It's an unknown factor, not a particular talent.

It's origin is related to algebra, where in a simple formula, if there is a single unknown, it usually gets designated as an x. "The X factor" therefore becomes "the one thing we do not conclusively know", but has lately been used to mean "an indescribable quality. We know it's good, we just can't put it into words."

As to the source of why the X factor as opposed to another letter; that's based on algebra.

Edit minor update to answer.

The X part meaning unknown has been covered fairly well here; factor, not so much. The word has two common meanings. The first is multiplicand, a component of a multiplication or product. From JHCL's Tolstoy quote, we an determine the strength S of an army of mass M via the equation

S = M*x

as soon as we can figure out what x is. I don't think this helps much with the query.

But factor also has a medical use, meaning some chemical -- a mineral, a hormone, a vitamin, etc. -- required for the operation of some physiological process. These appear, among other places, in the context of growth (of the body or some part thereof) and in blood coagulation.

Back in the day (i.e., the 1920s) doctors thought they had isolated a bacillus that caused influenza. They named it Haemophilus influenzae, the blood-loving influenza-causing bug. (They had indeed identified a pathogen, but not the one causing influenza, which is virus and was beyond the technology of the times to isolate.) Researchers determined that Haemophilus influenzae loved blood because it required two components of blood to survive. One, thought to be a vitamin or a vitamin analogue, they called V-factor. The other was a complete unknown, and that one they called X-factor.

The 1918 influenza pandemic hadn't yet been erased from the country's collective memory, and the return of the disease in the 1928 epidemic must have increased public awareness of it and the attempts to eradicate it. In its January 21, 1930 edition, The New York Times ruminated on the 3000-fold increase in US small pox cases over the previous year while noting the 40-fold decrease in that disease in Italy during the five years after the armistice:

It is the same X factor which so regularly arises to sober man's
pride in his conquests, to remind him that his bulletins of victory
one section of the front are to be read in light of less cheerful
information elsewhere.

Did X factor escape from a medical lab into common usage, meaning an unknown and mysterious but crucial element in the explanation of some phenomenon? The Times has no other references, and the V-factor shows up too late in that paper (on April 16, 1939).

In the 1942 book Mimsy Were the Borogoves an adult character (Holloway) describes a new kind of skill from the future which two child characters (Scott and Emma) are able to learn as the "x factor".