In Microsoft 365 compliance, sensitivity labels (Microsoft Purview Information Protection) are the feature designed to classify and automatically protect data. Microsoft states that sensitivity labels "let you classify and protect your organization's data" and that a label "can apply protection settings such as encryption" to files and emails. Labels can be deployed so that protection is applied without user action: Microsoft explains that labels "can be applied automatically, recommended, or by users," and that administrators can "auto-apply a sensitivity label to content in SharePoint, OneDrive, and Exchange based on conditions such as sensitive info types, keywords, or trainable classifiers." When a label with protection is applied, "encryption settings travel with the content" and enforce usage rights like who can open, print, or forward, even outside the organization.
By contrast, Content Search and eDiscovery are discovery/investigative tools and do not enforce protection.
Retention policies manage how long content is kept or deleted, but they don't encrypt content. Therefore, the Microsoft 365 compliance capability that encrypts content automatically based on specific conditions is sensitivity labels with auto-labeling policies in Microsoft Purview.