In the case of an indexer cluster, the number of copies of data that the cluster maintains. A cluster can tolerate a failure of (replication factor - 1) peer nodes.
On a multisite indexer cluster, a special version of the replication factor, known as the site replication factor, determines not only the number of copies that the entire cluster maintains but also the number of copies that each site maintains.
In the case of a search head cluster, the minimum number of copies of each search artifact that the cluster maintains.
For more information
In Managing Indexers and Clusters of Indexers:
In Distributed Search: