Reliability is a quantity which is measured by a number of factors. These factors may include Uptime, usage of resources, support service, enough storage capacity and bandwidth, and most importantly the relief from technical snags which keeps you busy in expanding your business instead of dealing with and fixing the unnecessary stuff.
Uptime is one of most important thing of any website hosting company to be considered as reliable. Let's just say if you are running an e-commerce website and you often encounter with downtime, it would just ruin your business and you would not ever put the tag of reliability to your web hosting company.
Same is the case with CPU Resources; to earn the tag of reliability, any web hosting company must not run its servers over capacity. That would simply antagonize the clients who by the way would get into competition with others on the same server for fair amount of resources.
RAID protected storage, Daily Back-Ups and Bandwidth also plays the vital role for any web hosting company to be in list of most reliable web hosting service providers.
Round the Clock Customer Support is one of main reasons clients like me would consider a highest factor for giving the certificate of reliability to any web hosting company.