QuickTime

QuickTime is Apple's technology for synchronizing audio and video. The name QuickTime refers to both a software library and an open file format. While the majority of QuickTime files are compressed using proprietary / closed-source codecs, the QuickTime  format itself is open.

Be sure to vote on the FeatureRequests page if you want Audacity to be able to open QuickTime  files (Mac and Windows only, sorry).