AGC or Automatic Gain Control is the term used to describe the function of automatic control of the radio-frequency gain.
In superior shortwave receivers, the signal is decoupled in front of the demodulator stage and used to control the gain of the RF preamplifiers via a signal amplifier (in some cases, this control voltage is also used to control the S-meter).
The regulation speed can be set to different time constants in high-quality receivers. For AM reception, a fast time constant is selected („AGC fast“), for the reception of Morse code (CW) a longer time constant („AGC Slow“). This prevents that during pauses between the dots and dashes, the amplification is increased; there is no carrier that could be used to control the amplification factor.
When the AGC is switched off or the switch is in position MGC, the radio frequency gain is controlled manually with the RF Gain control.