The World Wide Web Consortium defines synchronized media as;
"audio or video synchronized with another format for presenting information and/or with time-based interactive components, unless the media is a media alternative for text that is clearly labeled as such."
What this means is that synchronized media can be audio or video that is matched up with any other timed media. This can include video lined up with a separate audio track. This does not include media that acts as an alternative for text such as accompanying sign language video.
Indiana University has excellent definitions of both synchronized media and time-based media.
The examples below have been pulled from their article on "Making Video and Audio Accessible" found here.
"Synchronized media: Audio or video synchronized with another format for presenting information or with time-based interactive components. Examples include a video with a synchronized audio track or a slideshow with a synchronized prerecorded soundtrack"
"Time-based media: Live or prerecorded media that has time as a component, such as an audio recording, a video, or audio or video combined with interaction."