1. Broadly, any process that aligns a number of separate temporal sources so that they run in unison. Such sources are considered to be ‘in sync,’ but if they drift out of alignment they are ‘out of sync.’
2. (post-synchronization) In film or video post-production, the process of taking separate moving images and sound recorded at the same event and matching them together. In editing parlance, sync is any picture source that has synchronized sound—most typically footage of speaking subjects.