What is the reliability of a semiconductor device?

JIS defines reliability as follows.

"Characteristics that enable an item to perform its required functions under the stipulated conditions and for the stipulated time period"
(Japan Industrial Standard Z 8115 "Reliability Terms")

To put it simply, it means the "characteristics that enable a product to function without faults for a specific usage period" and refers to the time-related quality of the product.
Parameters used for the quantitative measurement of reliability, include reliability level, failure rate, and mean time to failure.

Reference document:
Review of Quality and Reliability Handbook Chapter 2