A unit load is defined as 100 mA in USB 1.x and 2.0, and 150 mA in USB 3.0. A device may draw a maximum of five unit loads from a port in USB 1.x and 2.0 (500 mA), or six unit loads in USB 3.0 (900 mA). There are two types of devices: low-power and high-power. A low-power device (such as a USB HID) draws at most one-unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws, at most, the maximum number of unit loads the standard permits. Every device functions initially as low-power (including high-power functions during their low-power enumeration phases), but may request high-power, and get it if available on the providing bus.

What is a unit load defined as?