Rectifier
A rectifier is the circuit that turns alternating current (AC) — which swings positive and negative many times a second — into one-directional current that a battery charger or DC power supply can use. It is the first stage inside almost every wall adapter: the 50/60 Hz mains has to be 'straightened out' before it can light an LED or charge a phone. The workhorse is the diode, used as a one-way valve that only passes the part of the wave it likes.
The simplest half-wave rectifier blocks one half of every cycle, wasting it; the far more common full-wave bridge uses four diodes so that both humps of the AC wave are flipped to the same polarity, doubling the useful output and halving the ripple. A smoothing capacitor then fills the gaps between humps, turning the bumpy output into a near-steady DC voltage.
Both half-cycles contribute, so ripple frequency is twice the line frequency.
The leftover wobble on the DC output is called ripple; a bigger smoothing capacitor reduces it but draws sharp current spikes at each peak, which is one reason cheap adapters can buzz and pollute the mains.