What is the origin of 4K term? According to Wikipedia:


  The name “4K resolution” refers to a horizontal resolution of approximately 4,000 pixels.


However, I believed that 4K is called like this simply because it has four times the resolution of a 1080p video, like this:



and, well, 1080 is practically 1024, which is a round number 1K.

Personally, the Wikipedia's explanation doesn't make much sense to me: the resolution of a piece of video is usually specified by two things: height and ratio; it is very unusual to see any mention of the width.

So what is the real origin of “4K” term?
According to Wikipedia ,Digital Cinema Initiatives originated the term 2K in 2002 for a digital film format of 2048 x 1080.  2048 is commonly referred to as 2K in the digital world.  It logically follows that when DCI went on to specify a resolution 2x larger in each direction, they called the 4096 x 2160 format 4K.  4096 is commonly referred to as 4K in the digital world.