Final answer:
The unused portion of a Master Clip in video editing is called the handle, which provides additional footage for transitions or minor adjustments during the editing process.
Step-by-step explanation:
The unused portion of a Master Clip is known as the handle. When editing video, a Master Clip is the original, unedited footage from which editors pull their selections. When part of this clip is used in the final edit, any segment that is not included in the edit and is left over, either at the beginning or the end, is considered the handle. These handles can be important for transitions or adjustments in the editing process. For instance, if an edit needs to be extended or shortened by just a few frames, the editor can use the handle rather than having to re-edit the entire sequence.