A point-in-time record of a backup set — every file exactly as it was at that moment.
A snapshot is a point-in-time record of a backup set — every file, folder, and permission exactly as it existed at that moment.
A snapshot is not a copy of your drive. It is a manifest: a list of paths pointing at content-addressed data in the repository, plus metadata like modification times, extended attributes, and Finder tags. Two snapshots taken an hour apart typically share 99% of their underlying data; only the manifest and the handful of changed blocks are new. This is why the hundredth snapshot costs nearly nothing in storage — you are paying for the diff, not another full copy.
Snapshots are immutable once written. Nothing edits them in place. When retention policy decides a snapshot is no longer needed, it is deleted as a whole, and pruning later reclaims any data that no surviving snapshot still references. This immutability is what makes point-in-time restore meaningful: the snapshot from last Tuesday at 3pm is identical bytes to what was written last Tuesday at 3pm, or it is gone.
In macup, every successful backup run produces one snapshot per backup set, stamped with the device it came from. When you open the restore browser and scrub the timeline, each tick mark is a snapshot — pick one, and macup reconstructs exactly the tree of files that existed the instant it was taken, down to the permission bits on a Lightroom catalog.