downage

plural downages

1. countableuncountablecomputingAn instance or period when the system is down, or unavailable for use.

   […] downages, and other anomalies in operation.

   1969, Howard Jacques Greenbaum, A Simulator of Multiple Interactive Users to Drive a Time-shared Computer System, page 30

   1 more example

   During trials by early adopters, each problem is identified and rated for severity, from system downage (most severe) to misalignment of a report column (minor).

   2002, Charles V. Breakfield, Roxanne E. Burkey, Managing Systems Migrations and Upgrades, page 109

   Synonyms: downtime, outage