A state captured at one moment and held unchanged. A snapshot is true of a time and stops being true immediately.
Snap (Middle Dutch snappen, to seize) joined to shot (Old English sceotan, to shoot); first a hunting term for a quick shot without careful aim, later borrowed for a quick photograph and then for a captured state.
A state captured in a single instant and held unchanged — true of exactly one moment, and no longer true the instant after.