Zero-downtime refers to the practice of performing system updates, maintenance, or deployments without causing any interruption to the availability or functionality of a service. In a zero-downtime environment, users experience no noticeable service outages or disruptions, even as changes are made behind the scenes. This approach is critical for applications and services that require high availability, ensuring continuous operation and minimizing the risk of lost revenue or user dissatisfaction. Techniques to achieve zero-downtime often include load balancing, rolling updates, blue-green deployments, and robust failover mechanisms.