What is Snap shot in Oracle database?

Answer 1

A snapshot is a replica of a target master table from a single point-in-time. Whereas in multimaster replication tables are continuously being updated by other master sites, snapshots are updated by one or more master tables via individual batch updates, known as a refresh, from a single master site.

See the below figure:-

Snapshots also have the option of containing a WHERE clause so that snapshot sites can contain custom data sets, which can be very helpful for regional offices or sales forces that don't require the complete corporate data set.

Answer 2

A recent copy of a table or a subset of rows or cols of a table is called as snapshot in oracle. A snapshot is more useful in distributed computing environment. We can create snapshot in oracle using command create snapshot. We can specify refresh interval while creating snapshot.
Snapshot log table maintains a record of modifications to the master table in a snapshot.

If by "snapshot" you mean a full copy of the database in a consistent mode, then I would restore the database from a backup and recover it until the desired point in time. The Oracle recovery processes will take care of of the consistency (tracked by System Change Number or SCN). If you are using RMAN for backups and recovery, there is a "DUPLICATE DATABASE" command with a time clause that will make this relatively painless. On the other hand, if you're just looking to extract a few tables in a consistent mode I can think of two options:
1. Export the group of tables with the consistent=y option of the (older) exp utility
2. Use the newer expdp utility with the flashback_time option