uptime means uninterrupted system availability. It is a measure of the time a website (blog, portal, web application) has been up (available) without any downtime (failure).
Website uptime is usually displayed as time value (days, hours) or as a percentage - what part of a given time it has been working.
A percentage uptime is commonly included in web hosting contracts as the SLA.
Website uptime value shows the quality of a hosting service - the bigger the better. It should be as close to 100% as possible. Industry average annual uptime value is 99,8%.