[whaaaaat]

A band, on a metric, indicates a minimum and maximum safe value. For instance, if you alarmed when your latency is below 50ms or above 150ms, that'd be your band. Being out of band is being out of that 'safe' window.

It's a totally valid and common jargon phrase from the web services world, apologies though, I assumed that it was wider jargon than it turns out it is...