The word “X” has been in use since the 18th century. Although the origins of the word are unclear it is believed to have originated from criminal slang for something that was fast as lightening.
From
there, it moved into popular usage to describe quick things. In the
19th century, Gilbert Newton Lewis–an American chemist was the
first to apply the term “X” to technical endeavors. He used “X”
to stand in for the time it took light to travel one centimeter
(approximately 33.4 picoseconds).
After
Lewis’s application, the term crept into usage in other scientific
pursuits. In electronics an “X” is the time between alternating
power cycles (1/60th of a second for most systems). In computing, an
"X" is one tick of the system timer (as such, a computing
“X” is not a fixed unit of time but an OS/hardware dependent unit
of time). In astro and quantum physics, an "X" is the time
it takes light to travel one fermi (the width of a nucleon) and
amounts to 3 × 10^-24 seconds
Although
“X” has established itself in multiple scientific communities, it
was hardly constrained to use by the technically inclined. Anytime
you see an “X” Lube oil shop, “X” Pop popping corn you're
seeing a 300 year old word alive and well in modern use.
ID
X.
