Mattermost is an open-source team collaboration platform (often compared to Slack) focused on mission-critical security. It offers secure, channel-based messaging, file sharing, and integrated voice and video conferencing. Mattermost can be deployed on-premises or in private clouds to maintain data sovereignty. It is used by governments, defense, and enterprise organizations that require high security and compliance. The service emphasizes operational sovereignty (it enables DevOps ChatOps) and provides features like multifactor authentication and compliance integrations for regulated environments.