Ghost voltage is a term that you'll hear used in tech circles, and often incorrectly. Ghost voltage is the name of a very specific phenomenon, but I've seen it used variously to refer to failing under load, high resistance connections, and even simple open circuits. What does it really mean?
What we call ghost voltage is transient, seemingly sourceless voltage. It does, of course, have a source. You know how when current flows through a conductor, it produces a magnetic field? Well that magnetic field can, in turn produce voltage in another conductor. That produced voltage will then just hang out in that conductor, just like charge does in a battery or a capacitor, until it is provided a path to neutral.
Does this mean that ghost voltage can power a load? Not at all -- the moment there's a path to neutral, ghost voltage bleeds off instantly. So why is it that your voltmeter can detect it? That's because a standard voltmeter (or VAC function on any multimeter) is designed to provide as much impedance to current flow as possible. The intent is so that you can get a measurement while affecting the circuit as little as possible. This high impedance means that your meter won't bleed off the ghost voltage, instead reading it as some funny-looking number of volts.
There's an easy way to avoid being confused by ghost voltage, and that's to use a loading meter. As its name suggests, a loading meter is designed to act as a load in the circuit it's measuring, with relatively low impedance -- low enough to immediately bleed off ghost voltage and prevent you from being faked out. If you're measuring a 120 VAC circuit and you read 120 volts on your loading meter, then you know for a fact that one of your leads is on a valid line and the other on a valid neutral. No guesswork required.
The bottom line is that, when doing AC voltage measurements, you should always use a loading meter. There's simply no reason not to. It will give you more accurate readings, it will confirm whether a power supply is actually capable of passing current, and it will keep you from getting tricked by any spooky ghost voltage.
This short video will give you a show you a real-world ghost voltage situation: