A computer screenshot is trivial today. Using software or even simple button press of PRT SC on the keyboard. But, the terminology comes from the act of literally taking a picture of the screen — the television screen that is.

B&W televised demonstration of working steam engine recorded in the first grade classroom and broadcast as local news.

Using a hand held [Argus C3] film camera in 1958, the resulting capture lost contrast clarity as it was a copy of a copy. (a flatbed scanner was used to scan the original photographic print just posted here so a copy of a copy of a copy) The light would have to be right as the camera would not have had automatic exposure and any glare or refection on the TV screen minimized. Camera focus would be a consideration; pre WYSiWG. Though grainy, it was a resourceful method and the proud parental event was luckily/successfully recorded.

We have it easy. SAVE AS would have archived the entire video and WITH sound, but back in the day this screenshot example was high tech.