An image of what is being shown on a device's display at a given moment.
It's equivalent to taking a photo of a device's display, but more directly, via software directly within the device instead of requiring a separate camera. The resulting image file is a pixel-for-pixel representation of what was shown on the device's display at the moment it was captured.
The ability to capture and save a screenshot is a common feature of most OSes, including Android and iOS.
After the screenshot is captured, it is saved as an image file that can be kept and/or shared.
Modern OSes make it easy to immediately share screenshots, offering an easy way to share information from a source that doesn't have its own "share" function.
Enhanced versions of screenshots include: Screen Recording, the video equivalent of a screenshot, and Scrolling Screenshot, which captures longer vertical content all at once (that would otherwise require scrolling down and taking multiple screenshots).