In dubbing of movies and videos, the process of making sure the lip movements of the characters on screen match the words spoken on the soundtrack.

This is one of the facts to consider when dubbing a foreign-language movie, or when making sounds for characters in animated movies.

It is a factor that both makers of the visual content and the writers of dialogue need to consider. Often, dubbed movies are worse than originals because it's surprisingly hard to make dialogue that would match the lip movement and have the same idea. In these cases, either lip synch fails and it looks bad, or the idea isn't conveyed completely accurately.