Synchresis is a term Chion invented to describe audiovisual relations based on simultaneity; when there is simultaneity between sounds and images, they blend with each other, and it becomes difficult, if not impossible, to separate them.

Chion describes synchresis as a universal, physiological phenomenon resulting from the makeup of our nervous system and not from any cultural conditioning, and as such operates as spontaneously as any reflex. It consists of perceiving the concomitance of given auditory and visual events as a single integrated event, so long as they occur simultaneously. Because this phenomenon occurs without any conscious effort on the part of the viewer, it leads viewers to establish a relationship of close interdependence and ascribe to a single common source sounds and images that may essentially be very different, originate from different sources and realistically have little in common.