Flicker. It's a term that inspired the name of a social network for photographers (Flickr), but it is also an issue which can cause a video to have a slight brightness variation while playing.
It can concern real-time footage, but it's more frequent on image sequences that are used to create a time-lapse or a stop motion video.
When flickering occurs (stop motion and time-lapse)
The animation technique called stop motion consists of taking single pictures of a small set inside which plasticine characters are portrayed, with the crew moving them between each photo in order to simulate motion.
Given that many reflex and mirrorless cameras are not perfect concerning exposure, there might be very slight variations in brightness between one picture and the next. Individually, they do not bother, but when combining the sequences to create a video, the flicker becomes annoying.
For similar reasons, flicker also affects time-lapses. These kinds of videos are created by taking photos at fixed intervals which are usually from one to ten seconds. The images are then edited using software like Adobe After Effects which, finally, exports a speeded up video.
For the same reason flicker can occur with stop motion, unfortunately, it also happens with time-lapses.
Talking, instead, of real-time videos, a very frequent case of flicker occurs when fluorescent lights are used because they do not emit continuous light but turn on and off thousands of times per second, causing different exposures on individual frames.
In the latter case, you can successfully fix the problem in the shooting phase, using 25 frames per second (fps) if you are shooting in Europe, or 29.97 fps if you are shooting in the USA.