Wild track, also known as wild sound and wild lines, is an audio recording intended to be synchronized with film or video but recorded separately. Generally, the term "wild track" refers to sound recorded on location, such as sound effects gathered when the cameras were not rolling or extra takes of lines performed for audio only.
When only the sound is needed, not the image; for example, recording a scream that will be heard off-camera.
When it is impossible to get good sound and video in the same take; for example, when actors are in a situation (such as the middle of a field in wide shot) that makes boom recording impractical and no wireless microphones are available.
When a take was good visually but there was a sound disturbance, and repeating the entire take is impractical.
To obtain room tone (the background noise of a location) which is necessary for post-production sound editing.
To create a censored version of a vulgar or suggestive piece of dialogue for television or radio viewing in post-production. Specifically, a less-vulgar version of the same line may be transposed seamlessly over the original. A notorious example is the famous line from Snakes on a Plane: the televised version has Samuel L. Jackson's character utter, "I have had it with these monkey-fighting snakes on this Monday-to-Friday plane!"
Note that wild track is considered something of a "cheapie" solution to these problems, and a big-budget production is more likely to use studio-recorded sound in these situations, as its quality is more controllable and predictable than wild track.
Dubbing, Automated Dialogue Replacement, in which actors read their lines in a studio setting to match lip movements already filmed.
Voiceover, in which a voice track is used but not synchronized with onscreen action.
Foley, in which sound effects are created in a studio.
Scratch track, a temporary sound recording intended to be replaced later.
