user6250: 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?

user10931: 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.

6250: <p>According to Wikipedia:</p>

<blockquote>
  <p>The name “4K resolution” refers to a horizontal resolution of approximately 4,000 pixels.</p>
</blockquote>

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

<p><a href="https://i.stack.imgur.com/40NiN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/40NiN.png" alt="enter image description here"></a></p>

<p>and, well, 1080 is practically 1024, which is a <a href="https://en.wikipedia.org/wiki/1024_(number)" rel="nofollow noreferrer">round number 1K</a>.</p>

<p>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: <em>height</em> and <em>ratio</em>; it is very unusual to see any mention of the width.</p>

<p>So what is the real origin of “4K” term?</p>
 10931: <p>According to Wikipedia ,<a href="https://en.wikipedia.org/wiki/Digital_Cinema_Initiatives">Digital Cinema Initiatives</a> 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.</p>
