The Free On-line Dictionary of Computing (30 December 2018):

power supply unit
power supply
PSU

    (PSU) An electronic module that converts high
   voltage (110 or 240 VAC) alternating current mains
   electricity into smoothed direct current at the various
   differnt voltages required by the motherboard; internal
   peripheral devices, cheifly storage devices: hard disks,
   CD or DVD, floppy disks and external connections such as
   USB.  A PSU needs a high enough power output rating to
   supply all the devices connected to it and should output as
   little as possible electrical noise, both on the output wires
   and as electromagnetic radiation.

   See also uninterruptable power supply.

   (2007-09-20)