What is the term used to describe an unexpected period where a system is not operational?

The term used to describe an unexpected period where a system is not operational is "downtime." This refers specifically to the time during which a system fails to function normally or is unavailable for use, regardless of the reason. Downtime can result from various factors, including technical malfunctions, system failures, or other unforeseen issues.

While "maintenance" refers to scheduled work performed on a system to keep it operational or to improve its performance, it does not capture the aspect of unexpectedness implied in the question. The term "outage" might also seem relevant, as it indicates a disruption in service. However, it is often used in specific contexts, such as telecommunications or power, and usually denotes a broader situation rather than the singular, unexpected failure denoted by "downtime." "Lag" refers to a delay in the response time of a system, which is a different kind of issue altogether and does not indicate an entire system failure.

In summary, downtime is the most precise term for an unexpected period of non-operation in a system, capturing both the abruptness and the operational impact of the failure.