In a distributed search environment, a Splunk Enterprise instance that handles search management functions, directing search requests to a set of search peers and then merging the results back to the user.
A Splunk Enterprise instance can function as both a search head and a search peer. A search head that performs only searching, and not any indexing, is referred to as a dedicated search head.
Search head clusters are groups of search heads that coordinate their activities.
Search heads are also required components of indexer clusters.
For more information
In Distributed Deployment:
In Distributed Search:
In Managing Indexers and Clusters of Indexers: