Rectifier
A rectifier is a circuit or part that turns alternating current, which flows back and forth, into current that flows in only one direction. It is usually built from diodes or, in older equipment, vacuum tubes. Its output still pulses up and down, so it is normally followed by filter capacitors to smooth it.
Example: A phone charger plugged into a wall outlet uses a rectifier to turn the outlet's back-and-forth current into one-way current for the battery.