phantom load
n. The electricity consumed by a device when it is turned off.
Mr. Stewart says a big source of waste is the approximately 6 per cent of the electricity used in typical homes by devices that consume power even when turned off.

Energy experts have dubbed this waste phantom load. The most common culprits are VCRs, which constantly draw six or seven watts to run their electric clocks or instant-on features, along with answering machines, lights and telephones with the big cube-like plugs.
—Martin Mittelstaedt, “Slow down the meter to save on power bills,” The Globe and Mail, November 26, 2003
Phantom loads: Hidden energy loads, such as the clock on a coffee maker, video recorders on standby, computers, and television instant-on features, can result in high energy bills.

The consumption from these hidden phantom loads in the United States is said to equal the electricity use of Greece, Peru, and Vietnam combined. When an appliance is not in use, switch it off at the wall.
—“Energy savers the real stars,” The Christchurch Press, September 11, 2001
1989 (earliest)
If you use an AC powered (via the inverter) pre-amp and rotor, unplug them when not in use. Their transformers are always on. As an aside, the same can be said about some other "phantom loads" like instant on TV's and boom boxes — another electron saved!
—Brian Green, “It's Gotta Be Spring,” Home Power, June 01, 1989
Continuing with the ghoulish metaphors, devices that use a phantom load are also called vampires (2001) because, presumably, they suck power even when turned off. (I'm sure someone has used the phrase "undead" to describe such a device.)

Many thanks to Michael Welch for supplying the earliest citation from 1989.