What capability allows for a unified search across multiple Splunk environments?

Federated Search is the capability that enables a unified search across multiple Splunk environments. It allows users to run a search query that can access and retrieve data from different Splunk instances, whether they are on-premises or in the cloud. This feature is essential for organizations that have disparate environments and need to maintain visibility across all of them without needing to merge data into a single index.

By utilizing Federated Search, users can obtain a holistic view of their data, leveraging varying environments' unique data while maintaining the independence of each system.

Other options, while related to search functionality, do not focus on searching across separate, distinct environments in the same way. Distributed Search typically pertains to the ability to distribute search queries across different search peers within a single environment. Search Head Clustering is aimed at providing high availability and load balancing for search heads within a single cluster. Multi-tenant Search refers to the capability of serving multiple tenants or users within a single environment, but does not imply searching across different Splunk environments.