Downtime

<<<<<<<<<<<<<<<<<<<<<<<<<
This is to be read with Dec 24 article "Uptime"
Click Most Updated Discount Coupon Codes & My Personal Web Hosting Recommendations if you are interested in those.
Stay tuned
<<<<<<<<<<<<<<<<<<<<<<<<<

Downtime inverse of uptime — an expression of the proportion of time that a service is unavailable, or "down."
In the web hosting industry, what constitutes a system being down can vary widely. It may indicate that a server is disconnected from the network entirely, or powered down. Alternately, it might simply mean that an HTTP service is shut down, thus rendering all web serving unavailable.
Downtime can be either scheduled or unscheduled downtime.

Scheduled downtime

Scheduling downtime is a common event among service providers, allowing them to undertake essential system maintenance that might otherwise interrupt service to their customers.

Scheduling downtime also allows customers to make alternative arrangements to ensure business continuity during the downtime period.

Unscheduled downtime

Unscheduled downtime generally occurs after an unforeseen, catastrophic event. This may include network failures within the infrastructure provider or simply a software failure within a system.
Software failure tends to be the most common cause of unscheduled downtime, ranging from operating system failures through to application level failures.

Source : http://www.webhostingtalk.com/ permetted republishing here