An image resolution of 3840 pixels wide by 2160 pixels high. In terms of video capture, content, and display, it is also known as Ultra HD (UHD).
It is named for the nearly 4,000 pixels in the width dimension. (Earlier formats such as 1080p and 720p measured the pixel height instead.)
See: 1080p
Compared to 1080p HD, 4K has about twice the resolution in each dimension, for four times the total pixels (8 megapixel, compared to 2 megapixels for 1080p.)
The next major step above 4K is 8K, which is nearly 8,000 pixels wide.