A measure used to indicate how long a system (e.g. a computer) has been up and running without going down. Without sufficient details, the measure is useless as an indication of quality or stability since other circumstances also influence the quality or stability of a system.
A system up for 3 years, hooked on the Internet, with a unsecure kernel or HTTPd is arguably of less quality than one which has been patched during these 3 years. A system which had hardware upgrades is arguably of better quality than e.g. one which makes noise because no hardware replacements were done.
The distinction between "criticial downtime" and "scheduled downtime" (see downtime) is a better indication of quality of stability.