A state of being not connected, obvious as it sounds. It can be used to describe physical or electrical disconnection (as in "the phone was disconnected from the wall" -- i.e. it was not plugged in).

However, it's also very useful for expressing a certain metaphorical state of mind, where one does not feel completely in touch with one's surroundings. You can talk about being disconnected from reality; which happens sometimes when you don't get enough sleep, or you're going through some seriously shitty jet lag (say, flying from Australia to Europe); or you're just depressed.

You can talk about being disconnected from someone else, where you just can't seem to bridge that interpersonal gaps and the interaction with that person remains at that uncomfortably, overly cordial, lots-of-gaps-in-the-conversation phase.