I take "downtime" to mean anything which prevents normal operation in any link of the chain supporting that operation, be it software, data, process, hardware, network, internet, 3rd party, power, etc.

It also has lesser severity levels in partial, decreased, delayed, etc.

An operational specification or expectation should specify the limitations, usually governed somewhere close to the level of patience and stress the end users and or business will accomodate, which varies between markets and types of business.

For example, MyFamily.com could probably go down for a couple of days and not lose many customers. But a trading company would probably not out-live the same... the latter threshold is measured in milliseconds and seconds, not hours or days.