A reliable server in web hosting is defined by its uptime, performance, and security features. It should maintain high availability, ensuring minimal downtime for websites hosted on it, typically striving for an uptime of 99.9% or greater. Performance depends on hardware capabilities and network speed, enabling fast loading times for users. Security measures, such as firewalls, anti-malware, and regular updates, protect against cyber threats. Additionally, reliable customer support and easy scalability are critical attributes, ensuring that clients can efficiently address issues and adapt to changing demands, enhancing overall user experience.