Term used to describe a service that is not available, temporarily or otherwise. Most commonly found when dialing a phone number, in which case usually preceded by three tones, each slightly increasing in frequency, each about half a second long, followed by a (very commonly female) voice saying one of several informational phrases. Common ones are "the number you have dialed is not in service, please check the number and try your call again", and "your call did not go through. please hang up now". If you're lucky, you'll get "the number xyz-123-4567 has changed to xyz-890-1234, I repeat.." or similar.