2160p resolution, often referred to as Ultra High Definition or UHD denotes a display resolution of 3840 pixels horizontally by 2160 pixels vertically.
The term "2160p" could be applied to any format with a height of 2160 pixels, but it is most commonly used in reference to the 4K UHDTV resolution of 3840 × 2160 due to its association with the well-known 720p and 1080p HDTV formats.
To summarize and compare, a display can be both true 4K and 2160p (4096 × 2160).
How Is 4K Different From UHD and 2160p? - Help Desk Geek
When shopping for a new TV, monitor, projector, or camera system, you’ve likely come across the terms 2160p resolution and 4K. These two terms are often used interchangeably, but are they actually the same thing? And does choosing one over the other make a difference in how you experience content?
Among the highest widely available standards today is 2160p, more commonly known as 4K resolution. It has become the benchmark for high-definition visuals, offering a sharp, lifelike viewing experience that far surpasses traditional HD or Full HD formats.
Projectors are a little more fiddly when it comes to the resolution quality, and currently the highest they can usually go is 2160p (the 3840 x 2160 pixels resolution).
While these terms may seem interchangeable, there is a subtle difference between the two. 2160p refers to a resolution of 3840 x 2160 pixels, while 4K refers to a resolution of 4096 x 2160 pixels. However, despite the technical differences, the term “4K” is often used interchangeably with 2160p.
2160p vs 4k: Why is it Called 4K Instead of 2160p ... - Home Theater Geek