Resolution measures the number of pixels in a digital image or display. It is defined as width by height, or W x H, where W is the number of horizontal pixels and H is the number of vertical pixels. For example, the resolution of an HDTV is 1920 x 1080.
A digital photo that is 3,088 pixels wide by 2,320 pixels tall has a resolution of 3088 × 2320. Multiplying these numbers together produces 7,164,160 total pixels. Since the photo contains just over seven million pixels, it is considered a "7 megapixel" image. Digital camera resolution is often measured in megapixels, which is simply another way to express the image resolution.
Every monitor and screen has a specific resolution. As mentioned above, an HD display has a resolution of 1920 x 1080 pixels. A 4K display has twice the resolution of HD, or 3840 x 2160 pixels. It is called "4K" since the screen is nearly 4,000 pixels across horizontally. The total number of pixels in a 4K display is 8,294,400, or just over eight megapixels.
Monitor resolution defines how many pixels a screen can display, but it does not describe how fine the image is. For example, a 27" iMac 5K display has a resolution of 5120 x 2880 while an older 27" Apple Thunderbolt Display has exactly half the resolution of 2560 x 1440. Since the 27" iMac is the same physical size as the Thunderbolt Display but has twice the resolution, it has twice the pixel density, measured in pixels per inch, or PPI.
NOTE: Unlike monitor resolution, the resolution of a printer or scanner is analogous to pixel density rather than total pixels. Printer and scanner resolution is measured in dots per inch, or DPI.