A Splunk Enterprise instance that indexes data, transforming raw data into events and placing the results into an index. It also searches the indexed data in response to search requests.
The indexer also frequently performs the other fundamental Splunk Enterprise functions: data input and search management. In larger deployments, forwarders handle data input and forward the data to the indexer for indexing. Similarly, although indexers always perform searches across their own data, in larger deployments, a specialized Splunk Enterprise instance, called a search head, handles search management and coordinates searches across multiple indexers.
The indexer is sometimes referred to by more specific terms, according to its context.
For more information
In Managing Indexers and Clusters of Indexers:
In the Distributed Deployment Manual:
In Distributed Search: