High availability (HA) refers to the ability of a system or application to remain operational and accessible with minimal downtime, even in the face of hardware failures, software issues, or other disruptions. HA is a critical consideration for systems that require continuous operation, such as online services, financial systems, and mission-critical applications. Achieving high availability typically involves implementing redundancy, failover mechanisms, and load balancing to ensure that if one component of the system fails, others can take over without impacting the overall availability of the service.

To build a high-availability system, architects often use strategies like clustering, where multiple servers work together to provide service, and data replication, where data is copied across multiple locations to prevent loss in case of failure. Additionally, HA systems are designed with monitoring and automated recovery processes to detect and respond to issues quickly. While high availability is essential for many organizations, it comes with additional costs and complexity, as it requires careful planning, robust infrastructure, and continuous testing to ensure that all components function correctly under various failure scenarios.