1. To keep a new system or device running continuously so that any weak elements or components will fail early and can be found and corrected before the system becomes an integral part of the user’s work routine. Such a test is often performed at the factory before a device is shipped.
2. To make a permanent change in the phosphor coating on the inside of a monitor screen by leaving the monitor on and keeping a bright, unchanging image on the screen for extended periods. Such an image will remain visible after the monitor is turned off. Burning in was a danger with older PC monitors; it is no longer a concern with most new PC monitors. Also called: ghosting.

The continuous operation of a device (as a computer) as a test for defects or failure prior to putting it to use
A long term screening test (either vibration, temperature or combined test) that is effective in weeding out infant mortalities because it simulates actual or worst case operation of the device, accelerated through a time, power, and temperature relationship. burn in

To increase the density of (portions of a photographic print) during enlarging by giving extra exposure.