High availability in relation to servers refers to a system's ability to remain operational and accessible without interruption. It is achieved through redundant systems, load balancing, and failover mechanisms to ensure services are consistently available even during hardware failures, maintenance, or unexpected outages. High availability is often quantified as a percentage, indicating the amount of downtime that can be tolerated, with goals such as 99.9% uptime, which is critical for businesses that rely on continuous access to data and services.