Transients are tiny spots which occasionally occurred in the film emulsion of photographic sky surveys taken prior to 1957 when satellites were first launched. They are called transient because they're only seen in one ~hour-long photographic exposure.

https://en.wikipedia.org/wiki/Aligned,_Multiple-transient_Ev...

https://www.metabunk.org/threads/transients-in-the-palomar-o...