What is the "Power Good Signal", according to the ATX specification?

What is the "Power Good Signal", according to the ATX specification?

• -5 Volt current the motherboard generates to indicate it's functioning.
• LED power light on the computer indicating proper function
• Means that the power supplied to the computer power supply is ample
• +5 volt signal generated in the power supply that is supplied to the motherboard 

EXPLANATION

The Power Good signal (sometimes called Power_OK or PWR_OK) is a +5 V (nominal) active high signal (with a variation from +2.4 V through +6.0 V generally being considered acceptable) that is supplied to the motherboard when the power supply has passed its internal
self-tests and the output voltages have stabilized. This typically takes place anywhere from 100 ms to 500 ms (0.1–0.5 seconds) after you turn on the power supply switch. The power supply then sends the Power_Good signal to the motherboard, where the processor timer chip that controls the reset line to the processor receives it.

SOURCE

http://www.tomshardware.com/reviews/power-supply-specifications-atx-reference,3061-2.html
https://en.wikipedia.org/wiki/Power_good_signal