Specifically, a way of recording sync sound where the camera runs at correct speed with a quartz crystal-governed motor, and tape recorder records its pilottone using a built-in quartz crystal pilottone generator. The crystal is much like the kind used in a quartz watch. Unlike cable sync, the camera and tape recorder are not attached.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission