PLAIN-LANGUAGE DEFINITION
Lip sync
Lip sync is the alignment of visible mouth movement with speech audio.
IN PRACTICE
An example
A generated presenter may animate its mouth to match a supplied spoken line.
What this does not tell you
Inspect the whole clip for timing and facial artifacts rather than judging one frame.
Use the term precisely
When you use this term in a brief or report, include the relevant source, scope and assumptions. An example explains the concept; it is not a benchmark or a promise of results.